(1 point) Determine which of the following functions are onto. OA.f: R R defined by f(x) = x3 + x. B.f:R? Rdefined by f(x, y) = (x + y, x - y). c.f: R R defined by f(x) = x2. D.f: R2 R2 defined by f(x, y) = (x + y, 2x + 2y). E.F : R R defined by f(x) = x