Which one of the following gives the component of the velocity to satisfy the continuity equation?

Question

In a two – dimensional fluid  flow u = 6x +xy. Which one of the following gives the component of the velocity to satisfy the continuity equation? 

         (a) 6x + xy          (b) 6 + xy        (c)  - (6y + xy)             (d)  - (6y+1/2y²)