请教新概念英语4上的一个填空题,关于CAN 和BE ABLE TO 的用法.

请教新概念英语4上的一个填空题,关于CAN 和BE ABLE TO 的用法.
he ____easily swim across this river if he wanted to(could)
amundsen ____reach the south pole before Scott(was able to)
i rang up several times before i ____contact him(was able to)
i ____get these tickets because i was willing to stand in the queue for hours(was able to)
这个问题是不小心发重了的，谁来随便回答一下马上给采纳