Hi @Madelyn-Luskey, thanks for your post. We are happy to discuss this question; no video is necessary.
The stimulus presents a set of facts; there is no conclusion, so it is not an argument. Let's break it down. Fact #1: 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. Fact #2: Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation. Fact #3: During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.
What does this mean? Well, Fact #3 tells us that during an ice age, a smaller concentration of oxygen 16 water ends up in the ocean. Why? Because, Fact #1 tells us, water vapor that evaporates from the ocean has a greater proportion of oxygen 16. Normally, per Fact #2, this evens out, because the evaporated water returns to the ocean via precipitation. But not in an ice age - then, the evaporated water is trapped as ice, rather than returned to the ocean.
Taking these facts together, you can select answer choice (B), which is textually supported and must be true. In a typical ice age, the concentration of oxygen 18 in seawater is increased, as water containing a larger proportion of oxygen 16 evaporates from the ocean and gets trapped in ice.
Hope this helps! Please let us know if you have any additional questions.
