The ship sank ,but fortunately he ______ to the

如果指在过去的某一个具体场合某人成功地做成了某事,一般不用could,而用were(was) able to
如：
The fire spread quickly throughout the hotel.Fortunately,all the people in it were able to escape from it.
注意,它们的否定式没有区别,如：
He was so drunk that he couldn't (wasn't able to) find the front door.
请在客户端右上角评价点“满意”即可

a 表示能力
b 中could表示可能性。