After years of research, he found the key ____ differences between species.
#5237
A
to explain
B
to explaining
C
of explaining
D
explaining
推荐题目
One is not born a ____; one becomes a _____.
#6644
A
genius; talent
B
talent; genius
C
genius; genius
D
gift; talent
  .Last summer I had ______ in the south of France .
#6668
A
amusing experience
B
a amusing experience
C
an amused experience
D
an amusing experience