please prove this using definition for the "if and only if" statement.

This is about Riemann Integral applications. Define increasing functions a, B, and y on [-1, 1 ] by a = X(o.1 1, B = X10.1 ], and y = (a + B). Given f E B[-1, 1 ], show that: (c) f E Ry [-1, 1 ] if and only if f is continuous at 0. (d) If f E Ry [-1, 1 ], then _, f da = _, f dB = _, f dy = f(0)