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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 is the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.

这个句子什么意思啊！尤其是carbon-14 dating不懂。

205. The peaks of a mountain range, acting like rocks in a streambed, produce ripples in the air flowing over them; the resulting flow pattern, with crests and troughs that remain stationary although the air that forms them is moving rapidly, is known as "standing waves."

crests and troughs是什么意思啊

dating指的是‘确定年代’

crests and troughs－波峰和波谷。