Match the functions $f$ with the plots of their gradient vector fields labeled A-D.

A. 
B. 
C. 
D. 

1.  $f(x,y) = x y$
2.  $f(x,y) = x^2 + y^2$
3.  $f(x,y) = x^2 - y^2$
4.  $f(x,y) = \sqrt{x^2+y^2}$