Let $O$ be the origin, and $\overrightarrow { O X } , \overrightarrow { O Y } , \overrightarrow { O Z }$ be three unit vectors in the directions of the sides $\overrightarrow { Q R } , \overrightarrow { R P }$, $\overrightarrow { P Q }$, respectively, of a triangle $P Q R$.
$| \overrightarrow { O X } \times \overrightarrow { O Y } | =$
[A] $\sin ( P + Q )$
[B] $\sin 2 R$
[C] $\sin ( P + R )$
[D] $\sin ( Q + R )$