They were surprised that a child should work out the problem____ they themselves could not. A. once...
2022-11-30 08:17:15 170次 2016届吉林省白山市高一上学期期末考试英语试卷（解析版） 单项填空 反馈错误 加入收藏 正确率 : 100%
题目内容：
They were surprised that a child should work out
the problem____ they themselves could not.
A. once B.
unless C.
while D.
if
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