The maximum frequency of X-rays produced by 30 kv electrons is

Question

$\begin {array} {1 1} (a)\;7.24 \times 10^{20}\: Hz & \quad (b)\;72.4 \times 10^{20}\: Hz \\ (c)\;7.24 \times 10^{18}\: Hz & \quad (d)\;72.4 \times 10^{18}\: Hz \end {array}$