They were surprised that a four-year-old boy should work out such a difficult problem they

发布时间：2020-08-16 07:32:46

They were surprised that a four-year-old boy should work out such a difficult problem
    they themselves couldn’t.
A. once
B. then
C. while
D. if

网友回答

A. once
B. then
(答案→)C. while
D. if
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