After years of research, he found the key differences between species.
A. to explain
B. to explaining
C. of explaining
D. explaining
网友回答
A. to explain
(答案→)B. to explaining
C. of explaining
D. explaining
解析：考查动名词做宾语：the key“关键”后面接to doing做定语，因为to是介词，所以接动名词，句意：多年的研究后，他发现解释物种间差异的关键。选B。