Robert is said abroad, but I don’t know in which country he is.A. have been studyingB. to have been studyingC. have studiedD. to have studied
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be said后应该接不定式形式，而且根据I don’t know in which country he is 意味着他现在仍然在国外学习，表示从过去到现在一直在做某事，要用完成进行体。所以选B，不能选D.