---May I go and play with Tom this afternoon, Mum?---No. You can't go out ________ your homework is being done.A.untilB.sinceC.beforeD.if
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B解析句意：--妈妈，今天下午我可以去和汤姆玩吗？--不行。你不可以出去，因为你在写作业。since引导原因状语从句。此题易误选A。状语从句中的时态是解题关键，选A应改为You can't go out until your homework is done.你把作业做完才可以出去。