The math problem seems very easy, but in fact it is very difficult .
A. to work out
B. being worked out
C. working out
D. to be worked out.
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(答案→)A. to work out
B. being worked out
C. working out
D. to be worked out.
解析：考查非谓语动词。句型S（人，物）+ be + adj. + to do，句子的主语在逻辑关系上为不定式动作的宾语，to do中的动词是及物动词时，不用被动，不及物动词后要接介词。句意：这道数学题看起来非常容易，但事实上却很难解出来。故A正确。