They were surprised that a child should work out the problem they themselves could not.
A. once
B. unless
C. while
D. if
网友回答
A. once
B. unless
(答案→)C. while
D. if
解析：考查连词：A. once 一旦B. unless除非C. while当…时候，虽然，然而，D.if如果，句意：他们很惊讶的发现，一个孩子可以算出这个问题，而他们自己不能。选C。