It is answer A. because if you add (+) a negative (-) number to another negative number it becomes positive.
this strikes out answer B. which is a negative subtracting 1 from itself, which makes -2 instead of zero (0).
Because of this theorem, you know you will need to add (+) or subtract (—) a the same numbers to or from each other to have the number zero (0).
this theorem strikes out answer D. which is different numbers subtracting from each other.
So the answer is A. because it is the same numbers adding (+) onto one another.