If f : (x, y) → (x + y, 2y – x) for every coordinate pair in the xy-plane, for what points (x, y) is it true that f : (x, y) → (x, y)?
(A)(0, 0) only (B)The set of points (x, y) such that x = 0 (C)The set of points (x, y) such that y = 0 (D)The set of points (x, y) such that x = y
A