Which of the following pairs of velocity components u and v satisfy the continuity equation for a two dimensional flow of an incompressible fluid?

A

$$u = (2x^2 + 3y^3); v = -3xy$$

B

$$u = (3x - y); v = (2x + 3y)$$

C

$$u = A \sin xy; v = -A \sin xy$$

D

$$u = cx; v = -cy$$