The professor explained this theory so clearly that this phenomenon was easy enough .
A. for us to work out
B. working out
C. for working out
D. to working out
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(答案→)A. for us to work out
B. working out
C. for working out
D. to working out
解析：考查不定式的符合结构。形容词easy后面可以使用不定式的复合结构。It is easy for sb to do sth.对某人来说做某事很容易。