Two types of earthworm, one black and one red-brown, inhabit the woods

A and B are good answers, but C is better.

A strengthens the argument because it says the red worm population is falling. One possible explanation is the black worms are now better camouflaged so the birds are eating more red worms.

But A isn’t ideal because there are other plausible explanations. Maybe there is a disease that’s killing red worms. We don’t know. A is tempting us to assume that the blackening of the forest is why the red worm population is falling but we don’t really know that for sure. So maybe this fact strengthens, or maybe it’s just a coincidence.

B is good, too, but it isn’t perfect. It says birds prefer black worms so you you would expect them to eat more black worms. Yet, the black worm population is rising. So we’re tempted to assume the birds would rather eat black worms but they aren’t doing so because the black worms aren’t visible so the birds settle for eating red. Okay, maybe. Or maybe the birds are still eating tons of black worms. We know the population of red and black is now equal but we don’t know that there are more black worms. Maybe they’re equal entirely because the population of red worms fell. Is that because the birds can see the red worms and are eating more of them? That’s a fair supposition but maybe something else, like disease or climate change, is causing the red worm population to fall.

And that takes us to C. C is best because it is ruling out these alternative explanations. A and B are plausible explanations, but they leave other possibilities open. C forecloses those possibilities.

This one is really hard, I would not fault anyone for picking A or B. We see a lot of questions where all the answers are bad and we have to find the least bad. Here, we have to pick the best of three pretty good answers.

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