Use the given constraints to determine whether each choice could equal zero. A: If x = 0, then xy = 0. Eliminate A. B: If x = 3, and y = −3, then x + y = 0. Eliminate B. C: If x = 3, and y = 3, then x − y = 0, Eliminate C. D: Factor: xy−y2 = y(x − y). For y(x − y) to be zero, either y must be zero or x − y must be 0; y cannot be 0, but x − y = 0 if x = y. Eliminate D. E: When a value is raised to an even exponent, the result must be greater than or equal to zero. Thus, y2 > 0, and x2 ≥ 0. The sum of y2 and x2 is thus greater than 0.