They were surprised that a child should work out the problem____ they themselves could not.
A．onceB．unlessC．whileD．if
C

試題分析：考查連詞：A. once  一旦B. unless除非C. while當(dāng)…時候，雖然，然而，D.if如果，句意：他們很驚訝的發(fā)現(xiàn)，一個孩子可以算出這個問題，而他們自己不能。選C。
點評：連詞的辨析要放在上下文總進行，同時要注意一些形式相近的連詞的詞義辨析。While的用法較多，也是常考的連詞。
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