A block of mass $m$ is placed on a smooth inclined wedge ABC of inclination $\theta$ as shown in the figure. The wedge is given an acceleration '$a$ ' towards the right. The relation between $a$ and $\theta$ for the block to remain stationary on the wedge is:

1. A
   $a=g\cos \theta$
2. B
   $a=\frac{g}{\sin \theta }$
3. C
   $a=\frac{g}{\text{cosec}\theta }$
4. D
   $a=g\tan \theta$