A line makes the same angle $\theta$, with each of the $x$ and $z$-axis. If the angle $\beta$, which it makes with $y-axis$, is such that $\sin^2 \beta = 3 \sin^2 \theta$, then $\cos^2 \theta$ equals
( A ) $\frac{2}{3}$
( B ) $\frac{3}{5}$
( C ) $\frac{1}{5}$
( D ) $\frac{2}{5}$