[求助] 一道lsat逻辑题

yuyuwang

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?

KEY: (B) The concentration of oxygen-18 in seawater is increased.

Why?

Thank you!

snow_mountain

雨水里的氧-16回不到海水里面。海水里面的氧-18不是就越来越多了吗？