关于THAT, 究竟在什么样子的从句中一定要有什么之中可以省略?

困惑....

比如说　ＯＧ　１５７

A number of linguists contend that all of the thousands of languages spoken by the world’s five billion people can be traced back to a common root language.

(A) that all of the thousands of languages spoken by the world’s five billion people can be traced

(B) that the world’s five billion people speak thousands of languages of which all can be traced

(C) the world’s five billion people speak thousands of languages which are all traceable

(D) all of the thousands of languages spoken by the world’s five billion people to be traceable（A）

(E) the ability to trace all of the thousands of languages that are spoken by the world’s five billion

A, the best choice, correctly (1) uses a noun clause introduced by that after contend

D incorrectly substitutes an infinitive clause for the “that” noun clause required after contend.

怎么会事？

ＰＳ，　我搜索过连接了，　没有讨论这个方面