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 people
A, the best choice, correctly (1) uses a noun clause introduced by that after contend, (2) keeps the “contention” clear by making all of the thousands of languages the subject of the noun clause, and (3) precisely indicates the relationship of the thousands of languages to the common root language (they can be traced back to it). B and C produce convoluted and ill-focused sentences by making the world’s five billion people the subject of the noun clause. The phrase of which all in B is unidiomatic (all of which is the idiom). C uses the wordy and indirect traceable back to. D incorrectly substitutes an infinitive clause for the “that” noun clause required after contend. E, in substituting a noun phrase, becomes incoherent and ungrammatical.
为什么traceable back to是wordy and indirect.
如果将A. that all of the thousands of languages spoken by the world’s five billion people can be traced改成：
that all of the thousands of languages spoken by the world’s five billion people are traceable | 
