please help me to answer this finance math problem. Thanks

3. Let r and o both be stopping times for the N-period binomial model. (a) Recall that the binary operation A : R2 -> R is defined by a Ab = min(a, b). Show that T A o is a stopping time in the N-period binomial model. (b) Define a binary operation V : R2 -> R by a Vb = max(a, b). Show that T V o is a stopping time in the N-period binomial model