GMAT Critical Reasoning (CR) | Re: Water vapor evaporated from the ocean contains a greater

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 asice.

Which one of the following conclusions about a typical ice age is most strongly supported by the statements above?