# A line AB in three-dimensional space makes angle $45^{\circ}$ and $120^{\circ}$ with the positive $x-axis$ and the positive $y-axis$ respectively. If AB makes an acute $\theta$ with the positive $z-axis$, then $\theta$ equals
( A ) $75^{\circ}$
( B ) $30^{\circ}$
( C ) $60^{\circ}$
( D ) $45^{\circ}$