A potential difference applied to an - ray tube is $5kv$ and the current through it is $3.2 \;mA$ Then the number of electrons striking the target per second is

Question

$\begin{array}{1 1} 2 \times 10^{16} \\ 5 \times 10^{18} \\ 1 \times 10^{17} \\ 4 \times 10^5 \end{array} $