PT1.S3.Q11 - The right answer still seems suspect...

My understanding is that in normal times, water vapor from ocean contains a heavier proportion of oxygen-18. However, if that water vapor is not retuning to the Ocean during ice ages, but getting trapped in glaciers, wouldn't that mean the ocean has LESS o-18? I guess I'm assuming the water vapor is taking O-18 out of the ocean. But even if that's not assumed, what warrant do we have to say the ocean has more O-18 than usual?