If n=1 and (m-n)/p=2 then which of the following is not a

If n=1 and \frac{m-n}{p}=2 then which of the following is not a possible value of m?

A. -2
B. -1
C. 0
D. 1
E. 2

This is Q 36 from Total GMAT MATH, I don't get the explanation provided in the book!

BB please!