My newly- purchased bag is nowhere to be found.____ I have left it on the bus?

What he said at the meeting made Mary rather embarrassed, so he felt that he ______ differently.

—Is that boy Alan?—No, it ______ be him. He left for Beijing yesterday.

It has been announced that all the candidates______ remain in their seats until all the papers have been collected.

Charlie ______ be really difficult at times even though he’s a nice person in general.