Seems, i got problem with it.
(1) x=8, y=5, then 8-5>8+5 wrong
x=8, y=-5, then 8--5>8-5 right
so A looks unsuff
from x-y>x+y we have y<0 suff
Looks like (B)
If you know that x=8, you know that y has to be negative!!
--> Because: x-y>10
If x is 8 and some number is subtracted and you get a number >10, the subtracted number has to be <-2
Hence you CANNOT choose 5 for y as you did!
Hope this helps.