Shouldn't the answer be C ?
Otherwise how would u know that x+y not equal to 0 ?
I remember a similar question in one of the challenges.
x+y must be different from 0 because the equation exists and is stated
... It's true that in the real GMAT, it will be stated that x+y != 0 to avoid any confusion.
We just have to remove a solution to the equation that does x+y=0 if such a solution exists.
Here, we arrive to x-y = 4 and we should keep in mind that the intersection of the 2 lines x-y = 4 and x+y = 0 must be removed.... But for this problem, we do not care it