An electron moving at a speed of $5 \times 10^6\: ms^{-1}$ is shot through a sheet of paper which is $2.1 \times 10^{-4}\: cm$ thick. The electron emerges from the paper with speed of $2 \times 10^6\: ms^{-1}$ . The time taken by the electron in seconds to pass through the paper sheet is :

Question

$\begin {array} {1 1} (1)\;5 \times 10^{-12} & \quad (2)\;6 \times 10^{-13} \\ (3)\;3 \times 10^{-12} & \quad (4)\;5 \times 10^{-13} \end {array}$