I don't know who invented ____ telephone,it's really ____most useful invention.

问题描述:

I don't know who invented ____ telephone,it's really ____most useful invention.
填冠词,请详细解释一下,谢谢了
第一个空是the, 第二个空是a,为什么?

这句话的意思是:我不知道谁发明了电话,但是它确实是最有用的发明
名词前加the指一类,a most=very