Obviously replying to you late, but I also chose D.

It's wrong because it does strengthen the argument. You're right that an alternate explanation would be an attack to the argument (and therefore fall under the except column we're asked about). And if it had just been an alternate explanation, D could have been a correct answer, but D doesn't just introduce an alternate explanation, it refutes one. It says "hey yeah you might be thinking that the population is declining because their habitats are shrinking, but nope. Habitats are the same size. Go fish" which is why it strengthens the argument. It shoots down the alternate explanation.

Hope this ramble helps someone.

Been trying to understand this concept for 9 months and JY just made it all make sense in 8 minutes and 13 seconds...

404 thought process not found

Wow I was way off. I was focused on the level of understanding: fully/completely vs. properly. I really liked AC E but went with D (I know, let the booing commence) because E using such strong language as "understand completely" seemed too far from the premise.

My reasoning in choosing C was that something being Hierarchical was not sufficient for concluding anything. It's the necessary component in the argument. But I guess something could be concluded, just not with what we've been given..

At first I thought of C more as "we don't know the percentage of gasoline in gasohol so even if we're burning more gasohol than we would have gasoline on its own, we still might not necessarily be consuming more gasoline as a whole" but this is still irrelevant because regardless, this choice does not strengthen the argument. When using the trick of reading the argument and inserting "and" before each answer choice to see if it strengthens that argument, this one doesn't hold up.