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

1. $a=\frac{g}{\cos ec\theta }$

2. $a=\frac{g}{\sin \theta }$

3. $a = g\cos \theta$

4. $a = g\tan \theta$