下面这一题你会做吗?

The fire spread through the hotel quickly but everyone __________ get out.

A. had to                        B. would                        C. could                         D. was able to

此题应选D。容易误选C(could)。这里主要涉及could与was able to的用法和区别：

1. 表示过去一般性能力时 (即某人想干什么就能干什么能力)，两者都可用：

He could [was able to] run very fast when he was a boy. 他小时候跑得很快。

2. 表示过去特定的能力 (即某人在某一具体场合做某事的能力，往往暗示经过努力才具备的能力)，通常用was(were)able to，也可用managed to do sth或succeeded in doing sth，但是不能用 could：

他学习很努力，所以考试及了格。

正：He worked very hard and was able to [managed to] pass his examination.

误：He worked very hard and could pass his examination.

正因为此原因，所以上面一题的答案用 ...was able to get out，而不用...could get out，因为此题谈及的是一种特定场合的具体能力。(www.yygrammar.com)

但值得注意的是，以上用法只适合于肯定句，若在否定句中，则可用couldn’t代替 wasn’t [weren’t] able to：

He worked hard but wasn’t able to[couldn’t] pass the exam. 他学习很努力，但考试却未能及格。