五道初三英语词转题.

根据所给内容,用其正确形式填空.（求详细解析!）

The Grand Canyon is one of the greatest  ________（nature)  wonders of the world.

It's amazing that the baby is still  _______(live)  in the accident!

During this year,we learned a lot of _______(knowledge)  from the teachers.

In fact,there are many _______(difference)  between the twin brothers although they look the same.

You should give up ________(smoke)  because it has done great harm to your health.

附1-3题答案：1.natural（为何这样变形?） 2.alive 3.knowledge（为何不变复数?）

4、5两题我对老师给我的答案存疑,附：4.differentes 5.smorking

不只求答案,更求大神的解析!