53. It can be inferred from the passage that precipitation formed from evaporated ocean water has
(A)
the same isotopic ratio as ocean water
(B)
less oxygen 18 than does ocean water
(C)
less oxygen 18 than has the ice contained in continental ice sheets
(D)
a different isotopic composition than has precipitation formed from water on land
(E)
more oxygen 16 than has precipitation formed from fresh water
答案(B)，没有问题。但是怎么理解(C)呢？
OG的解释：C is incorrect because the passage suggests that the water evaporated from the ocean contributes to the growth of continental ice sheets, which should therefore have an isotopic composition similar to that of the precipitation ice sheets, which should therefore have an isotopic composition similar to that of the precipitation formed from evaporated ocean water.
文章的相关描述：When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it.Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces,the remaining ocean water becomes progressively enriched in oxygen 18.
从文章中可以得到这些信息：ice age 时期，1、蒸发的水最终会回到ocean中；2、水蒸发的时候，重一点的 isotopes蒸发的少，留在ocean的多。由这两个信息可以得到(B)，没有问题。
但是怎么比较precipitation和the ice contained in continental ice sheets的isotopic ratio呢？文章似乎没有关于the continental ice sheets 的isotopic ratio的信息啊。 | 
