They kept trying _____ they must have known it was hopeless.
He ______ have sent the e-mail at an earlier time, but there was something wrong with the network.
—Can Professor Li be in the laboratory?—He ______ be; I saw him there half an hour ago.
My newly-purchased magazine is nowhere to be found. ______ I have left it on the train?
—Did your mother sleep well last night?—She ______ have slept through all those noises.
Don't use water to put out the fire if oil catches fire. As is known, oil ______ float on water.