Let $L$ be the line of intersection of the planes $2x + 3y + z = 1$ and $x + 3y + 2z = 2$. If $L$ makes an angle $\alpha$ with the positive $x-axis$, then $\cos \alpha$ equals :

Question

( A ) $1$
( B ) $1/\sqrt{3}$
( C ) $1/2$
( D ) $1/\sqrt{2}$