Which of the Following Utility Functions Is an Example of Preferences

Multiple Choice

Which of the following utility functions is an example of preferences for perfect substitutes?

A) $U ( x , y ) = y \sqrt { x }$
B) $U ( x , y ) = \min \{ 2 x , y \}$
C) $U ( x , y ) = 3 x + 5 y$
D) $U ( x , y ) = 2 x ^ { 2 } + 4 y$