Q.An electron moves in a straight line inside a charged parallel plate capacitor of uniform charge density $\sigma$. The time taken by the electron to cross the parallel plate capacitor undeflected when the plates of the capacitor are kept under a constant magnetic field of induction $\vec B$ is [the four answer options in the source scan mix the symbols $\varepsilon_0$, $\sigma$, $e$, $l$, $B$ in a layout that could not be verbatim-transcribed with confidence -- held rather than guessed]