Oxygen-18 is a heavier-than-normal isotope

Answer choices

1. A

   Correct26% picked this

   Once it is formed over the Atlantic, the rain cloud contains more ordinary

   Why this is right

   This is supportable from the second sentence. In a rain cloud, water molecules with oxy-18 are rarer than water molecules with normal oxygen.

   Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.

   26%
2. B

   Out of Scope: greater than normal18% picked this

   Once it has passed over the Amazon, the rain cloud contains a greater-than-normal

   This is practically contradicted by the fact that as the cloud passed over the Amazon, its oxygen-18 "remained fairly constant". Assuming the rain cloud started out with an average percentage of oxy-18 (we have no reason to assume these rain clouds were atypical), it still had an average percentage, since its oxy-18 remained fairly constant.

   18%
3. C

   Percent vs. Amount26% picked this

   The cloud’s rainfall contains more oxygen-18 than

   The 3rd sentence is saying something pretty confusing. During rainfall, a higher proportion of the oxy-18 than normal oxygen in a cloud's "bank account" falls out. But since there is less oxy-18 in the cloud to begin with, a higher percentage of a lower total could still be a smaller amount than a lower percentage of a higher total. Pretend you and I both went to Las Vegas. I brought less money to gamble with than you did. In the first few hours, I lose a higher percentage of my money than you lose of yours. But you still may have lost more money. If I brought $100 to gamble and lost 50%, that's not as much as if you brought $1000 to gamble and lost 25%. (The oxygen-18 is me, the normal oxygen is you, and the rainfall is the money we've lost so far.)

   26%
4. D

   Contradicted Too Strong: must15% picked this

   During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as

   It seems like this answer is wanting to bait people into thinking that "because there's less oxy-18 in the cloud but a higher percentage of it falls out during rainfall" that clouds are somehow trying to balance things out? This is contradicted by the 3rd sentence, in which we learn that clouds are surrendering different percentages of oxy-18 vs. ordinary oxygen.

   15%
5. E

   Too Specific16% picked this

   During a rainfall, the cloud must surrender more of its oxygen-18

   Too Specific: more than half its oxy-18 All we know is that a higher proportion of the oxy-18 falls out than does the normal oxygen. But we don't know that a cloud, during rain, always loses more than half of its oxy-18.

   16%