Determine which of these relations are reflexive. The variables $x$, $y$, $x'$, $y'$ represent integers.

A. $x \sim y$ if and only if $x + y$ is odd.
B. $x \sim y$ if and only if $x + y$ is positive.
C. $x \sim y$ if and only if $x - y$ is positive.
D. $x \sim y$ if and only if $xy \geq 0$ .
E. $x \sim y$ if and only if $x-y$ is a multiple of 10.