At a Consumer's Interior Optimum Solution, Which of the Following $M U _ { x } = M U _ { y }$

At a consumer's interior optimum solution, which of the following will not necessarily hold true?

A) $M U _ { x } = M U _ { y }$
B) $\frac { M U _ { x } } { M U _ { y } } = \frac { P _ { x } } { P _ { y } }$
C) $M R S _ { x , y } = \frac { M U _ { x } } { M U U _ { y } }$
D) $M R S _ { x , y } = \frac { P _ { x } } { P _ { y } }$