Yeah, I see what you're saying. This is a mean question. 😣
With question #7, they don't mention the combustion train AT ALL, so you basically have to pretend you haven't read the paragraphs that involve the combustion train. Instead, they just mention the propane, oxygen, and water, which means we should only be referring to the balanced chemical equation in the passage. From there, we can see that we have oxygen as the limiting reactant (this is rare!) and our fuel in excess. Then babityboop, we can determine B is the answer.
With question #8, THEY MENTION THE COMBUSTION TRAIN 😮😮😮🚅🚅🚅🔥🔥🔥. So, we need to incorporate everything they've said about that in the passage. That's the big difference! Now that we have oxygen in excess (thanks combustion train), then the fuel must be limiting. This is how combustion normally works.