The old couple have been married for 40 years and never once_____with each other.
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.