Q.Which of the following temperature will read the same value on Celsius and Fahrenheit scales. a. - 40° b. + 40° c. -80° d. -20°

Any quantitative measurement means comparing something against a reproducible, unchanging standard; the arbitrarily-fixed and universally-accepted standard used for this comparison is called a unit. Historically, several parallel unit systems were used side by side — CGS (centimetre, gram, second), FPS (foot, pound, second) and MKS (metre, kilogram, second). In 1960 the General Conference of Weights and Measures adopted a single, unified system, the International System of Units (SI), built on the decimal metric system that originated in eighteenth-century France. SI defines seven independent base physical quantities — length (metre), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole), and luminous intensity (candela) — from which every other unit (such as those for speed, volume or density) is derived by combination.