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必须阻止工厂向附近河流排放废水。
Factories _______ ______ ______ _______ pouring waste water into the rivers nearby．
Sandy必须打扫起居室。
Sandy has to ________ the ________ room.
Do you________ ________go so soon?
我必须提醒你学校的规章制度。
I________ ________ ________ ________the school________．
你必须及时把图书归还到图书室。
You must ________ ________ ________ ________ the library in time．