As for your other question:
If x-y>10, is x-y>x+y?
First, manipulate the second equation.
If x-y>x+y, then we can subtract x from both sides and add y to both sides and get:
Then divide by 2, and our question becomes whether y is negative.
(Remember, we CANNOT divide or multiply an inequality by a variable, since we don't know whether the variable is positive or negative, or even 0.)
Now, look at statement 1.
This tells us that y is negative, which is all we need to know.
Obviously this one is enough, as it also tells us that y is negative.
Thus, the answer should be D.
Hope this helps,