Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater. Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation. During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice. Which one of the following conclusions about a typical ice age is most strongly supported by the statements above? ['The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself.', 'The concentration of oxygen-18 in seawater is increased.', "During the ice age, more of the Earth's precipitation falls over land than falls over the ocean.", 'The composition of seawater changes more slowly than it does in interglacial periods.']