Which of the Following Utility Functions Best Represents the Idea $U ( x , y ) = \sqrt { x y }$

Which of the following utility functions best represents the idea that two goods,x and y,are perfect complements?

A) $U ( x , y ) = \sqrt { x y }$
B) U(x,y) = x + y.
C) $U ( x , y ) = | x - y |$ .
D) U(x,y) = min (x,y) .