__________ our earth, or else it will be no longer fit for us to live on.
| A．Protecting |
| B．To protect |
| C．Protect |
| D．Protected |
He ______ have sent the e-mail at an earlier time， but there was something wrong with the network.
| A．must | B．should |
| C．could | D．might |
—Can Professor Li be in the laboratory?
—He ______ be； I saw him there half an hour ago.
| A．can | B．must |
| C．might | D．need |
My newly-purchased magazine is nowhere to be found. ______ I have left it on the train?
| A．Could | B．Should |
| C．Must | D．Would |
—Did your mother sleep well last night?
—She ______ have slept through all those noises.
| A．mustn't | B．may not |
| C．can't | D．won't |
Don't use water to put out the fire if oil catches fire. As is known, oil ______ float on water.
| A．shall | B．must |
| C．may | D．will |