Your stimulus says the following:
Until now, only injections of vaccine were available.
These were primarily used by older adults.
Now nasal sprays are available that prevent flu in children.
Children are more likely to contract and spread flu.

Conclusion: If nasal sprays are made available for children, it will greatly reduce the spread of flu across the population.

Does something come to mind when you read this conclusion? What came to my mind was that if children are most likely to contract and spread flu, they should have been given flu injections and that would have prevented spread of flu across the population. Why is it that availability of nasal spray will prevent the spread the flu but injections did not?

Now, I need to strengthen the argument so I should focus on my conclusion and see what will strengthen it the most.
Let's read the options:
1. If a person has already been given an injection, he is immune to flu. Nasal spray on top will not help in anyway.
2. This says that nasal sprays work in the same was as do injections. It does not tell me why injections could not prevent the spread of flu while the nasal spray will.
3. This tells us that the subsidies have kept injections affordable for all older adults. But it doesn't say that otherwise their cost is prohibitive. Also, it doesn't say anything about the cost of nasal spray. If this option said, "Injections are very expensive otherwise but nasal spray is a cheap alternative", it might have made a stronger contender. Still, we do not know whether cost is a factor that parents consider at all when getting vaccines for their children. To make this option the answer, we might even have to add something like "Parents are not willing to get their kids immunized if the vaccine is very expensive"
4. Few older adults catch flu from children doesn't strengthen my conclusion that making nasal sprays available for children will reduce the spread of flu across the population.
5. Now that is what I was looking for! A reason why parents don't give flu shots to their kids but will be willing to give nasal sprays. Parents don't like to give shots to their kids (could be because of pain associated with a shot or whatever) Now, with the availability of the nasal spray, they will be more inclined to have their kids vaccinated. This probably will help in preventing the spread of flu across the population.
Answer (E).
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Isn't D a kind of assumption here ?

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