Nobody knows how people first came to these islands. They _____ from South America on rafts.
A .must have sailed　
B. can sail　
C. might have sailed
D. should have sailed
C
考察虛擬語氣。根據(jù)上文，沒有人知道人們第一次是怎么到達這些島上的。可知下文猜測的事情發(fā)生在過去以前，故需用過去完成時態(tài)。Must表示必須、一定；might表示可能；should表示應(yīng)該。根據(jù)上文可知不是很確定，故選C，他們可能成木筏從南美洲過來。
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