211. The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.
(A) has been established by carbon-14 dating, but what is much more difficult to determine are
(B) has been established by carbon-14 dating, but what is much more difficult to determine is
(C) have been established by carbon-14 dating, but what is much more difficult to determine is
(D) have been established by carbon-14 dating, but what is much more difficult to determine are
(E) are established by carbon-14 dating, but that which is much more difficult to determine is
Two instances of subject-verb agreement must be observed in this sentence: The period... has been established and what is much more difficult to determine... is. Both clauses have singular subjects and must have singular verbs. (what从句，ETS认为是单数，所以谓语用单数)WHAOnly B, the best choice, observes these requirements. （主谓一致）A incorrectly uses the plural form are in the second clause. Choices C and D incorrectly use the plural form have in the first clause, and D incorrectly uses are in the second clause as well. E incorrectly uses the plural form are in the first clause. Furthermore, because the date of the period in question was established before the writing of the sentence, the verb of that clause must be in theform (has been established).（Jnlvo：before the writing of the sentence对此句而言=before now. 所以present perfect form 。
Furthermore, because the date of the period in question was established before the writing of the sentence, the verb of that clause must be in theform (has been established).
确认时间是在写这个句子前 是过去的动作
但现在完成时 是过去的动作对现在的影响或持续到现在 这里如何能理解出这个动作会对现在有影响或持续呢
或者说用现在完成时unnecessary (ETS常用)
谢谢 | 
