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
网友回答
(答案→)A. for us to work out
B. working out
C. for working out
D. to working out
解析： 句意：教授解释的这个理论如此的清楚，以至于这种现象对我们来说足够容易解决。 easy后常用to do 结构，所以选A。