@AreeshaSaif

I don't think (C) gives an alternative explanation. The author's explanation is that the mild winter caused the larger bird population. The problem is that the first causal chain stops too early. It says Mild winter → More natural foraging → Fewer feeder visits. It doesn't explain how fewer feeder visits lead to larger bird population. (C) fills that gap by adding the missing link:

Fewer feeder visits → Fewer birds eaten by predators → More birds survive → Larger Bird Population. So it completes the missing link in causal chain. It doesn't provide an alternative explanation.

@AliMerhi Here we have Group 4 (necessary) and Group 3 (sufficient) indicators. What I understood is that when no and unless appear in the same sentence, using unless rule is usually less complicated.

In the argument, we have two ideas: Gryffindor (G) and bravery (B). Using the unless rule, I usually choose the idea that comes after unless, which here is bravery. we negate it and get:

/B → /G

Since Gryffindor is already negated in the sentence, the translation becomes

/B → /G

Then we take the contrapositive:

G → B

That matches what he got in the video. However, the conclusion says B → G, which confuses the sufficient and necessary conditions. The original statement only gives us G → B, so we cannot conclude B → G. That reversal is why the argument is not valid. I hope this makes sense.