The period when the great painted caves atLascaux and Altamira were occupied by Upper Paleolithic peoples has beenestablished by carbon dating, but what is much more difficult to determine arethe use to which primitive peoples put the caves, the reason for theirdecoration, and the meaning of the magnificently depicted animals. A. has been established bycarbon dating, but what is much more difficult to determine are B. have been established bycarbon dating, but what is much more difficult to determine are C. have been established bycarbon dating, but that which is much more difficult to determine is D. has been established bycarbon dating, but what is much more difficult to determine is (D) E. are established bycarbon dating, but that which is much more difficult to determine is
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答案D。但是to determine +be+ the use...the reason....and the meaning... (宾语是三个名词),为什么be要用单数?what引导的从句做主语谓语动词要怎样判断?请大牛解答。
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