It never _____ humans until the mid1990s that some animals can be cloned.
| A．occur | B．is occurred | C．occurred to | D．occurred |
—The similarities between social networks are striking.
—That ________ come as no surprise，because they are all products of the Internet.
| A．should | B．shall | C．may | D．must |
—Hi，Joan.Any idea where Tom is?
—He ________ on the playground.I saw him there playing football just now.
| A．shall be | B．should have been |
| C．must be | D．might have been |
—Guess what？I came first in that contest.
—Great！You ________ a lot during the past two weeks.
| A．must practise | B．should practise |
| C．should have practised | D．must have practised |
—Where will you start your work after graduation?
—Mum，it's not been decided yet.I ________ continue my study for a higher degree.
| A．need | B．must | C．would | D．might |
—Why do you look so puzzled?
—Oh，I ________ over a most difficult maths problem.
| A．thought | B．have thought |
| C．am thinking | D．think |