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

A．water molecules containing oxygen-18 condense and fall as precipitation slightly sooner
than those containing oxygen-16
B．the ratio of oxygen-18 to oxygen-16 in water vapor evaporated from oceans if different from that of these isotopes in seawater
C．growing ice sheets tend to lose their oxygen-18 as the temperature of the oceans near them gradually decreases
D．less water vapor evaporates from oceans during glacial periods and therefore less oxygen-18 is removed from the seawater