How many solutions does the system have?
6x-y=-1
6x+y=-1

This system only have one solution to the problem that is asked

Multiplication property of zero

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.