---______have the scientists been there？---For about two years．A.How many timesB.HowC.What timeD.How long
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D
解析分析：答句改为完整句的话就是The scientists have been there for about two years．由于是副词there，所以介词被省略了，还原的话应该是have been in(一直在某地)，for about two years就表示have been there的时间长度。该用how long．(延伸)“for+时间长度”在句子中担任状语，表示动作或状态的持续时间长度，用how long来提问。在It takes+时间，spend+时间的句型里，时间是句子中的宾语，提问时也用how long，如：It took him two hours to do the work．How long did it take him to do the work？