#14 - When polar ice caps grow (during ice ages, for example), lighter forms of oxygen

[Administrator]

Complete Question Explanation

Must Be True. The correct answer choice is (D).

Answer choice (A):

Answer choice (B):

Answer choice (C):

Answer choice (D): This is the correct answer choice.

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[lsathelpppp]

Can you explain why B is wrong and D is correct?

[Chandler H]

Can you explain why B is wrong and D is correct?

Hello,

Sure! Answer choice (B) is wrong because we only know that the ice caps grew for 20,000 years and then shrank for 10,000 years. However, we don't know the rate of growing or shrinking, so there's no way for us to know if they were larger at the beginning of that 30,000 year period or not. What if the ice caps grew from (for example) 5 million tons at the beginning of the period, to 15 million tons 20,000 years later, and then shrank back to 10 million tons 10,000 years after that?

Answer choice (D), on the other hand, just tells us that the ice caps grew for 20,000 years, then shrank. We know that because of the information in the stimulus about the sea shellfish. It's much more moderate than other answer choices, which is a good sign.

Does this make sense?

[lounalola]

why is C incorrect?

[Amber Thomas]

Hi Lounalola!

Let's break down our stimulus:

Premise 1: When polar ice caps grow, lighter forms of oxygen accumulate in ice caps, leaving greater concentrations of heavy forms of oxygen in the sea
Premise 2: This heavy oxygen is absorbed by marine organisms
Premise 3: As ice caps shrink, the concentrations of heavy oxygen in sea water decrease
Premise 4: Concentrations of this heavy oxygen in shellfish increased for 20,000 years, and then decreased for 10,000 years during a 30,000 year period

To pre phrase our answer. we know that heavy oxygen concentrations increase as ice caps grow, and decrease as ice caps shrink. Therefore, we can infer that for the 20,000 year period where the heavy oxygen concentrations were increasing, ice caps were growing, and for the 10,000 year period where heavy oxygen concentrations were decreasing, the ice caps were shrinking.

Now, let's look at Answer Choice C: "the beginning of the period coincided with the onset of an ice age that lasted approximately 20,000 years"

Our first sentence states: "when polar ice caps grow (during ice ages, for example)..."

This doesn't tell us that the ONLY circumstance where polar ice caps can grow is during an ice age, it just tells us that ONE of the circumstances where polar ice caps can grow is during an ice age. Therefore, we actually don't have enough information to infer that the 20,000 year period was an ice age-- we can only infer that for some reason, polar ice caps were growing. It COULD have been an ice age, but we can't confirm or deny that.

I hope this helps!