Thank you both. I think Rolex is on the right track. On the one hand, the other endings in the exercise have no comma, either, and they are also followed by a coordinate conjunction. On the other, the exercise is about the use of modal verbs.
I understand that using "be able to":
... but she was able to get over it soon.
you are speaking about past
ability.
However, you can also use
could for past
ability:
I could swim when I was three.
So, why not
could here?

One guess of mine is that perhaps in the first case (
be able to) we are not only speaking about ability, but also about an achievement, the completion of something, but I don't know whether I'm right there.

Any help?
Thanks
