how brave he was!he was able to swim across the river in such a bad weather

题目

how brave he was!he was able to swim across the river in such a bad weather
A.could B.was able to C.might D.should
为什么不选A?could不是也有能力的意思吗?

答案

语义上,can/could属于情态助动词,它指主语具备某种“潜在能力”,但并不包含该能力的实施；而be able to属于半助动词,它则除了指主语具备某种“能力”外,还往往暗示该能力的付诸实施.针对过去情况时,两者的区别意义明显,不可混用.因为,在过去情况下,主语“潜能”的实施与否,业已成了定局.比较：
1）We could save his life.
我们有办法救活他.（但未必实施）
2）We were able to save his life.
我们得以救活他.（付诸实施）
句1）只表示主语有救人的能力,但未必实施；句2）则表示主语非但有救人的能力,而且业已成功实施了.