The graph of which equation forms an ellipse?

(1)x^2 -y^2 = 9 (3) 2x^2 +y^2 = 8

(2) 2x^2 + 2y^2 = 8 (4)xy= -8

I tested myself on a textbook quiz and got this question wrong. I selected choice (2) but the correct answer is choice (3). What makes choice (3) correct?