Why this is right
It's very weak, lovable wording to hear "X cannot be determined by looking at Y alone". It is extreme to say "X can be determined by looking at Y alone", so it is weak to rule it out. If we see that global sea level has risen 2 inches, would that tell us that glaciers and ice sheets have melted the amount of water it takes to raise the sea level by 2 inches? No, because we know that these artificial reservoirs we've built are hiding some of the real increase. The rise in global sea level would be more than 2 inches if it weren't for these reservoirs. The melting of glaciers and ice sheets may have been enough to raise sea level 3 inches, but because we captured so much water in these reservoirs, it only went up 2 inches. Let's say we started a lemonade stand and started a bank account for all the revenue we make from that lemonade stand. You look at the bank account and see there's $1000 in there. Can you determine by looking at that number alone how much revenue we've made from the lemonade stand? No, not if told you, "That bank account balance is lower than it would otherwise be, since we loaned some money to our friend from that account so that he could start his own cupcake stand." Since this loan makes the bank account smaller than it otherwise would be, we can't determine how much revenue the lemonade stand brought in simply by looking at the bank account balance. Similarly, since these reservoirs make the sea level rise less than it otherwise would, we can't just look at the rising sea level and use that on its own to calculate how much new water there is from the melting of glaciers and ice sheets. Additionally, the expanding volume of water is contributing to the rise in sea level, so even if there weren't reservoirs, we couldn't attribute the full 2 inch rise to the melting of glaciers and ice sheets. Part of the rise is attributable to the expanded volume of warmer water. (We would need to also know how much that expansion contributes to the rise, in order to then determine how much of the rise came purely from melting).
Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.