gre考试阅读真题解析(A)_GRE阅读

　　18. The passage asserts that one reason that oceans become enriched in oxygen-18 as ice sheets grow is because

　　 water molecules containing oxygen-18 condense and fall as precipitation slightly sooner than those containing oxygen-16

　　 the ratio of oxygen-18 to oxygen-16 in water vapor evaporated from oceans is different from that of these isotopes in seawater

　　 growing ice sheets tend to lose their oxygen-18 as the temperature of the oceans near them gradually decreases

　　 less water vapor evaporates from oceans during glacial periods and therefore less oxygen-18 is removed from the seawater

　　 the freezing point of seawater rich in oxygen-18 is slightly lower than that of seawater poor in oxygen-18

　　19. According to the passage, the large ice sheets typical of glacial cycles are most directly caused by

　　 changes in the average temperatures in the tropics and over open oceans

　　 prolonged increases in the rate at which water evaporates from the oceans

　　 extreme seasonal variations in temperature in northern latitudes and in mountainous areas

　　 steadily increasing precipitation rates in northern latitudes and in mountainous areas

　　 the continual failure of snow to melt completely during the warmer seasons in northern latitudes and in mountainous areas

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