(This problem mainly uses the complete square formula. I don't know if I learned it in grade one. )

Let the original selling price be 1. (Omit below%)

A: (1+m) (1+n) =1+m+n+Mn.

B: (1+n)( 1+m)

c:[ 1+(m+n)/2][ 1+(m+n)/2]= 1+m+n+(m+n)2/4。

A and b are the same. Now we just need to compare mn with (m+n) 2/4 as the difference method.

(m+n)^2/4-mn=[(m+n)^2-4mn]/4=(m-n)^2/4≥0

Because m≠n, (m-n) 2/4 >; 0

Therefore, plan C raises the price the most.