That's a good question, and you are right about the existence of a causal argument in that example. That one presents an interesting mix of causal reasoning and numbers and percentages:
Exclusive tea drinkers' blood cells took half as long to respond to germs as exclusive coffee drinkers' blood cells.
In other words, the tea drinkers' blood cells were twice as fast as coffee drinkers' blood cells in responding to germs.
The author concludes that an immune system boost from tea must have given the tea drinkers the advantage, and caused this margin between the response times of the two groups.
- Cause Effect
Immune boosting tea tea drinkers faster than coffee drinkers in germ response time
The correct answer to this Assumption question is answer choice (C); the author must assume that drinking coffee didn't actually slow down
the coffee drinkers by half. If we apply the PowerScore's Assumption Negation technique, we logically negate, or take away, any given assumption to see whether that weakens the argument. When we negate this answer choice, we get the following:
- Drinking coffee did cause the coffee drinkers' blood cells' response time to double.
This could also have caused the tea drinkers' advantage with regard to germ response time:
- Cause Effect
Immune slowing coffee tea drinkers faster than coffee drinkers in germ response time
The author must assume that this alternative was not the actual cause of the margin in germ response time.
Good question, and an interesting one—I hope this is helpful! Please let me know whether this is clear—thanks!