If a vector $\overrightarrow P $ making angles $\alpha, \beta\ and\ \gamma$ respectively with the $X, Y$ and $Z$ axes respectively. Then ${\sin ^2}\alpha + {\sin ^2}\beta + {\sin ^2}\gamma = $
A
$0$
B
$1$
C
$2$
D
$3$
If a vector $\overrightarrow P $ making angles $\alpha, \beta\ and\ \gamma$ respectively with the $X, Y$ and $Z$ axes respectively. Then ${\sin ^2}\alpha + {\sin ^2}\beta + {\sin ^2}\gamma = $