Which of the following is a true statement regarding the indicated block cipher mode? Select two.

a$.$

GCM computes a MAC to ensure the message was not tampered with during transmission.

b$.$

CTR requires the control engine on the sender and receiver to be synchronized.

c$.$

Although ECB uses a basic approach, it is still among the most effective block cipher modes.

d$.$

CBC uses the output of a round of encryption as input for the next round.