Design a counter modulo 4 (sequential circuit with two flip-flops and one input U) which work like that:
1. When U=0, the state of the flip-flop does not change.
2. When U=1, the state sequence is code Gray 11, 10, 00, 01 and repeat.
a. implement with D flip-flops
b. implement with J K flip-flops
c. implement with T flip-flops
Specify how initialize the initial state of the counter