A: I read all the books on the list. B: You _________ - it wasn't necessary at all! We only had to read the first two.
needn't have
A: Jenny seemed upset on the phone last week. B: She had a cold. It’s quite possible/She ______ still felt really ill.
might/may have
A: Phil flew back from holiday yesterday. B: That’s impossible/He ________. There was a strike at the airport.
can't/couldn't have