Nobody knows how people first came to these islands. They must have come from South America on rafts.

admin2011-07-28  34

问题 Nobody knows how people first came to these islands. They must have come from South America on rafts.

选项

答案must——might

解析 前半句强调“无人知道”,因此后半句应当是不肯定的推测,用might have done。must have done是肯定推测,与句意不符。
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