问题

I ________ work out the problem at first, but after my teacher showed me an example, I ________ get the answer easily.
A:couldn’t; was able to
B:can’t; could
C:wasn’t able to; could
D:was able to; was able to
B:can’t; could
C:wasn’t able to; could
D:was able to; was able to
正确答案是: