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题库 / Prep2007E1-SC-136
It seems likely that a number of astronomical phenomena, such as the formation of planetary nebulas, may be caused by the interaction where two stars orbit each other at close range.
may be caused by the interaction where two stars orbit each other
may be caused by the interaction between two stars that each orbit the other
are because of the interaction between two stars that orbit each other
are caused by the interaction of two stars where each is orbiting the other
are caused by the interaction of two stars orbiting each other
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because of 不能跟在be后面，必须跟着一个normal verb；He do it because of money.
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