Q18. According to Bohr's theory, the moment of momentum of an electron revolving in $4 ^ { \text {th } }$ orbit of hydrogen atom is:\\
(1) $\frac { h } { \pi }$\\
(2) $\frac { h } { 2 \pi }$\\
(3) $8 \frac { h } { \pi }$\\
(4) $2 \frac { h } { \pi }$