The velocity componentsin the x and y directions are given by $$u = \lambda xy^3 - x^2y; v = xy^2 - \frac{3}{4}y^4$$ The value of $$\lambda$$ for a possible flow field involving an incompressible fluid is

A

$$-\frac{3}{4}$$

B

3

C

$$\frac{4}{3}$$

D

$$-\frac{4}{3}$$