Premise: A vitamin is a vitamin, no matter the source.
Conclusion: It is a waste of money to pay extra for vitamin pills that are made of higher-quality or more natural ingredients.
Let's try to poke some holes in this argument. Remember that vitamins and vitamin pills are not the same thing.
"Other than the vitamins contained, pills with natural ingredients have more dietary fiber than those with artificial ingredients. This is worth the higher price." "Capsule shells made from lower-quality ingredients are more likely to cause cancer than those made of more expensive, higher-quality ingredients. Do not be cheap when it comes to vitamin pill purchases."
I think these are pretty convincing arguments against the nutritionist's statement. In order to reach this conclusion, the nutritionist must have assumed something that would defend the argument from my attacks.
If answer choice E is assumed, then both of my counterarguments are invalidated. This is what makes E the correct answer. In assumption questions, sometimes the argument will have a vulnerability, and the right answer will protect this vulnerability.