[求助]OG 211中的时态问题

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
og关于E的解释：

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 the present perfect form (has been established).

og 的意思是说，因为时间被确立发生在写这个句子之前，所以必须用现在完成。

很多发生在之前的都是用过去时啊，完成时态好象只有1 一个动作发生在过去已经完成并对现在产生影响。2 一个动作发生在过去并有可能持续到现在。这里也没有完成事的标志时间提示~~~

对完成时态和过去时态一直搞不清楚,老是错,哪位NN能给我讲讲,万分感谢!

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