—Frank,look!Whoarethechildrenunderthetree

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—Frank, look! Who are the children under the tree __________ waiting in a line?

—They are the students from No. 1 Primary School.

A. that are B. where are C. which is D. who is

【解析】根据句意：——弗兰克，你看!树下排队的孩子们是谁？ ——他们是来第一小学的学生。可知先行词是children，指人，作主语，故选代词用who/that，先行词是复数，动词用are。故选A。