Q.Do you know for how many months pregnancy last in dogs, elephants, cats? Find out.

The four listed pairs correspond to a, b, c, d respectively.

1. (a) First month – Formation of heart. ✓ Correct. "After one month of pregnancy, the embryo's heart is formed."

2. (b) Second month – Movement of foetus. ✗ Mismatched. By the end of the second month the foetus develops limbs and digits. The first movements of the foetus (together with the appearance of hair on the head) are observed during the fifth month. So pairing month 2 with foetal movement is incorrect. …

Step 1 — The route of infection.

After entering the body, HIV first enters macrophages, where its RNA genome is copied into DNA by reverse transcriptase; this viral DNA integrates into the host cell's DNA and directs the production of virus particles. The macrophage keeps functioning as an HIV factory.

Step 2 — The key target cell.

HIV then enters helper T-lymphocytes ($T_H$ cells) — it does so because its envelope glycoprotein binds the CD4 receptor, and helper T cells are the CD4⁺ population. Inside them, the virus replicates and produces progeny viruses, which are released into the blood and go on to attack further helper T cells.

Step 3 — Why this matters clinically (the internal check). …

Step 1 — The three layers of the uterine wall

Step 2 — Why the endometrium is glandular

The endometrium is the functional, hormone-responsive lining:

• Under estrogen it proliferates (proliferative phase).
• Under progesterone from the corpus luteum its glands become secretory, storing glycogen to nourish an arriving blastocyst (secretory phase).
• It is the layer in which implantation occurs.
• If fertilisation fails, the corpus luteum degenerates, progesterone falls, and the endometrium breaks down and is shed — this is menstruation. …

Concept — Structure of the human spermatozoon (Human Reproduction). A sperm is a microscopic haploid cell made of a head, neck, middle piece and tail, all covered by a plasma membrane.

1. Head — contains an elongated haploid nucleus carrying the paternal genome. → s

2. Acrosome — a cap-like structure over the anterior part of the head, formed from the Golgi body and filled with hydrolytic enzymes (e.g. hyaluronidase) that help the sperm penetrate the zona pellucida of the ovum at fertilisation. → p

3. Middle piece — possesses numerous mitochondria arranged in a spiral sheath; these generate the ATP that powers the flagellar beat. → q …

The question asks about two specific developmental milestones: when the mother first feels the foetus move, and when hair begins to appear on the foetus’s head. These are standard landmarks in prenatal development, and they are often tested together because they occur at roughly the same time.

In human gestation, the first movements of the foetus — called "quickening" — are usually felt by the mother between the 4th and 5th month. However, the first clearly noticeable movements are most consistently reported around the 5th month (18–20 weeks). At this stage, the foetus is large enough and its muscles strong enough for the movements to be perceptible.

Simultaneously, the development of hair on the head (lanugo, which later becomes finer hair, and then true scalp hair) begins around the same period. By the end of the 5th month, fine hair is visible on the head, and this is the standard textbook milestone.

Let’s walk through the reasoning step by step.

1. First foetal movements

   The foetus starts moving much earlier (around 8–10 weeks), but these movements are too weak for the mother to feel. By the 5th month (18–20 weeks), the foetus has grown enough that its limb movements become strong and are felt by the mother. This is a classic milestone in obstetrics.

2. Appearance of hair on the head

   Hair follicles begin forming around the 3rd month, but visible hair on the scalp appears later. By the 5th month, the foetus has a fine layer of hair (lanugo) covering the body, and hair on the head becomes distinct. This is the accepted timing in standard biology curricula.

3. Checking the options

   • (A) 5th month: Matches both milestones. …

Step 1 — The definition. The NCERT chapter Organisms and Populations opens with exactly this idea: organisms that cannot tolerate a changing internal state regulate and thereby maintain homeostasis — constancy of the internal environment — even when the external conditions (temperature, salinity, pH, water availability) fluctuate.

Step 2 — Examples. Mammals and birds keep their body temperature near $37^\circ\mathrm C$ by sweating/shivering; the kidney holds blood osmolarity steady; insulin and glucagon hold blood glucose steady. Each is a homeostatic mechanism, and each works by negative feedback.

Step 3 — Why the other options are narrower or wrong. …

Region a — the head.

Contains the acrosome, a cap of enzymes that digests the egg's outer layers — its function is to help the sperm penetrate the ovum.

Region b — the middle piece.

Densely packed with mitochondria, spiralled around the axial filament — its function is to provide the energy (ATP) the tail needs to keep moving. …

The key here is to recall the sequence of organogenesis — the process by which organs form and mature in the developing foetus. Each month brings a new set of milestones, and the order is logical: the most vital organs (like the heart) form first, followed by the body's structure (limbs, digits), then surface features (hair), and finally fine details (eyelid separation).

Let’s match each month to its landmark event.

1. First month (i) → (c) Heart

   By the end of the first month (week 4), the heart is the first functional organ to form and begins beating. This is the earliest major organogenesis event among the options.

2. Second month (ii) → (d) Limbs & digits

   During the second month (weeks 5–8), the limb buds appear and differentiate into arms, legs, fingers, and toes. This is when the basic body plan takes shape.

3. Fifth month (iii) → (b) Hairs on head

   By the fifth month (around week 20), lanugo (fine hair) covers the body, and scalp hair begins to grow. This is a later developmental feature. …

The life cycle of Plasmodium (the malarial parasite) alternates between a human host and a female Anopheles mosquito. The critical distinction to remember is that sexual reproduction (gametogony and fertilisation) happens only inside the mosquito, while asexual reproduction (schizogony) happens inside the human. Many students mistakenly think fertilisation occurs in human blood because they see gametocytes there — but those gametocytes are merely the precursors; they must be taken up by the mosquito to complete the sexual phase.

Let’s trace the sequence:

1. In humans (asexual cycle)

   When an infected mosquito bites a human, it injects sporozoites into the blood. These travel to the liver, invade liver cells, multiply, and then burst out as merozoites. The merozoites invade red blood cells (RBCs), where they multiply further, causing the cyclical fevers. Some merozoites, instead of continuing asexual multiplication, develop into gametocytes (male and female) inside RBCs. These gametocytes circulate in the blood but do not fuse inside the human — they are dormant until taken up by a mosquito.

2. In the mosquito (sexual cycle)

   When a female Anopheles mosquito bites an infected human, it sucks up blood containing these gametocytes. The gametocytes enter the mosquito’s stomach (also called the midgut). Here, the change in environment (lower temperature, higher pH) triggers the gametocytes to mature into gametes. The male gamete (microgamete) fertilises the female gamete (macrogamete) inside the stomach lumen — this is the fertilisation event.

3. After fertilisation …

Step 1 — Locate the reaction.

After glycolysis, pyruvic acid formed in the cytoplasm enters the mitochondrial matrix. There it undergoes oxidative decarboxylation — the "link reaction" that connects glycolysis to the Krebs cycle.

Step 2 — The complete equation.

The reaction is catalysed by the pyruvate dehydrogenase complex, which requires several cofactors — notably Mg$^{2+}$ (shown over the arrow in the stem) and TPP.

Step 3 — Assign X and Y.

In the stem, the reaction is written as reactants $\rightarrow$ X $\rightarrow$ Y + CO$_2$ + NADH + H$^+$.

• X is the agent that acts on the reactants — the enzyme, pyruvate dehydrogenase.
• Y is the carbon-containing product that goes on to enter the Krebs cycle — acetyl CoA (the 2-carbon acetyl group carried on coenzyme A). …

Step 1 — Recall the adrenal gland.

Each adrenal gland has two parts: the outer cortex (secretes corticoids — glucocorticoids like cortisol, mineralocorticoids like aldosterone) and the inner medulla (secretes the catecholamines: adrenaline/epinephrine and noradrenaline/norepinephrine).

Step 2 — The emergency response.

Under stress, emergency or excitement, the sympathetic nervous system stimulates the adrenal medulla. The catecholamines released produce a coordinated set of effects that prepare the body to face or escape danger:

• increased heart rate, force of contraction and cardiac output
• increased rate of respiration and dilation of bronchioles
• glycogenolysis and lipolysis, raising blood glucose and free fatty acids to fuel muscles
• pupillary dilation, piloerection, sweating …