In some messages sent by Bob, the adversary may know the first block M1 and want to replace it by…

In some messages sent by Bob, the adversary may know the first block M1 and want to replace it by another block A1 of his choice, leaving the rest of the message unchanged. Show that the adversary can achieve this if Counter mode is used. Do you think he can do it with CBC mode (assume changing C0 = IV is allowed)?