We're slightly more than halfway through Section 2 of the June 2007 LSAT, and predictably, the questions have started getting tougher. If you're new to the LSAT, you should probably focus on the earlier questions in each section, which tend to be much easier. Where possible, I'll try to link the more difficult questions to an easier question of the same type. Number 15 turns out to be a Sufficient Assumption question. So far in this section, we've done two previous Sufficient Assumption questions: this one and this one. You might want to check out those two explanations first, if you haven't already. The conclusion of the argument says "every year it will be necessary for all high-risk individuals to receive a vaccine for a different strain of the virus." The support for this assertion is "each year's vaccination will protect only against the strain of the influenza virus deemed most likely to be prevalent that year." There's a hole here. Your most important job is to identify that hole.
The question says "Which one of the following is an assumption that would allow the conclusion above to be properly drawn." That's a Sufficient Assumption question. On this type of question, you're the attorney for whoever is making the argument. You're analyzing your client's argument, and figuring out what evidence is missing to prove your client's case. You're going to call one expert witness, who will say whatever you want them to say. The answer choices are going to be five different experts--you can only choose one of them to call to testify. Before you interview the prospective witnesses, what would the perfect testimony be? What fact would guarantee a win for your client?
This is a really formulaic process. The needed testimony is always a bridge between the existing evidence and the conclusion of the argument. I think the perfect missing testimony would be "any given strain of virus is only ever deemed most prevalent once." If that's true, and if each year's vaccination only protects against the one strain of virus deemed most prevalent that year, then it is inexorably true that everyone will have to get vaccinated every year (for that year's strain).
Now all I have to do is pick the expert who says exactly what I'm hoping he'll say, or something with the same effect:
A) Nah. Not what I'm looking for. What do the total numbers have to do with anything? The numbers can be the same or change, I don't care. I want to prove that everyone has to get vaccinated every year. This answer doesn't prove that.
B) Nope. Again, who cares? This answer doesn't help me prove that everyone has to get vaccinated every year. I'm laser-focused on my client's conclusion. If you don't help me prove my client's case, then you're fired.
C) This one is tempting, except it doesn't prove that everyone would have to get vaccinated every year--what if the same virus struck three years in a row? If that happened, then this answer wouldn't help me.
D) Boom. This is similar to my prediction, and has exactly the effect I was looking for. If this is true, then my client wins. Simple as that. You're hired.
E) Nah, side effects are totally irrelevant. Our answer is D.