Here’s what helped me:

1. High-iron group has more Parkinson’s.

2. Possible alternative explanation: maybe the high-iron group just contains more genetically predisposed people.

3. A says genetically predisposed people are not especially high-iron consumers.

4. Therefore, genetic predisposition is less able to explain why the high-iron group has more Parkinson’s.

5. That leaves iron itself as a more plausible explanation.

I know this example was being used to teach the third type of formal argument, but as was pointed out in the video, the main conclusion contains the negate sufficiency indicator word “unless”. So would it be appropriate and faster to apply the conditional indicator group 3 strategy instead?

As several have pointed out, “never” was excluded from the video as a negate necessary indicator word. What was the rationale?

You're younger than I.

I broke it down like this:

The most recent scientific research shows that

the proportion of fat calories is significantly lower in the diets of

people who read the nutrition labels on food products

than it is in

people who do not read the nutrition labels.

The first paragraph in this comment is the best explanation of this question. You narrow it down to A and D, then pick A because D has other problems. A is only correct because all the others are wrong. haha

There are a million other grab bag reasons, but I see now that it has to be that the study was too short because it is crystal clear in the stimulus that a long-term prediction is being made.