Water vapor evaporated from the ocean contains a greater proportion of oxygen 16 and a smaller proportion of the heav...

Hi everybody, I've noticed that a lot of people are confused by the wording in answer B. Many are reading it as meaning that the amount of Oxygen 18 is increasing. It is not saying that at all. Let's look at the wording very carefully. It states, "The CONCENTRATION of oxygen 18 in seawater is increased." While they're not 100% the same, the following words are all expressing relative amounts, so we can think of them as being similar: concentration = proportion = percentage What's happening in the ocean is that more O16 than O18 is leaving. Normally, the rain brings it all back, so the ratio stays the same. In the ice age, though, the water is not returned to the ocean - it is added to ice caps. That means that the total amount of water that is left in the ocean is decreasing. As more O16 is leaving, we could say that proportionally, more O18 is being left behind, thus skewing the original ratio by increasing the amount of O18 relative to the amount of O16. An example from real life might make it a little easier to understand. Let's say that I had 200 pens in a drawer, 100 red and 100 blue. If I took out 2 blue and 1 red at the beginning of the day, there would be one more red than blue pens in the drawer. At the end of the day, though, if I put them all back, the original ratio would be maintained. What would happen if I didn't replace the pens? There would be more red pens than blue pens, and the more days that passed, the greater the difference in number of red pens to blue pens. Note that the actual number of red pens isn't "increasing," though. The proportion of red pens to blue pens is increasing. Sorry for writing such a long post, but the concept of proportions can be hard to grasp.