When you have a CR problem that asks you to EXPLAIN A DISCREPANCY, it's usually in this format:

- It's generally true that ______.
- Normally, you'd think this would mean ______.
- In this case, though, _______.

We need to find an answer that
1) keeps the original conditions true, and
2) supports the surprising outcome.
Avoid answer choices that try to undermine the premises or support the expected outcome.

In this case:
It's generally true that less rainfall = slow rivers, and slow river = algae.
Normally, you'd think this would mean that drought = more algae.
In this case, though, extreme drought = low algae levels.

The key is to find an answer that addresses the difference that EXTREME drought makes versus normal drought conditions.

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
This supports the expected outcome - if algae-eaters decline, there should be MORE algae.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
This explains why the premise (slow water = high algae) is true, but doesn't explain the surprising outcome.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.
Other species are irrelevant

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
If the rivers dry up completely in EXTREME drought, then the rivers aren't just slow-moving - they don't exist anymore! The habitat that the algae lives in ceases to exist, so much of the algae would die. This keeps the original conditions true, and explains the surprising answer. Correct.

(E) Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.
Any answer that starts "except during extreme drought" can't possibly tell us what it is about extreme drought that makes things different. This also just supports the given premise: slow rivers = high algae. It does nothing to explain why extreme drought = low algae.

The answer is D.