I work out the problem at first, but after my teacher showed me an example, I get the an

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

(答案→)A. couldn’t; was able toB. can’t; couldC. wasn’t able to; couldD. was able to; was able to解析：句意:起初我不能做出那个问题，但是在老师给我做了例子后，我就很容易地得到了答案。can作情态动词的基本意思是“能,能够”“可以”“可能,会”,可表示体力、智力能够完成一件事情或环境赋予的能力; 也可表示由于环境或其他因素而产生的可能性,只有现在时和过去时两种形式,，它可表示不一定能够做完一件事；而be able to则有多种时态形式。有能力做，并且做完了事情。故选A。

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