问题
选择题
They were surprised that a child should work out the problem ________ they themselves couldn't.
A．once
B．then
C．While
D．if
答案
答案：C
while（implying a contrast）含相对之意，表示对比转折。
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They were surprised that a child should work out the problem ________ they themselves couldn't.
A．once
B．then
C．While
D．if
答案：C
while（implying a contrast）含相对之意，表示对比转折。