All of John's friends say they know someone who has smoked 40 cigarett

Hi,

in inference questions that ask for what "must be true?", it is a great strategy to deal with every statement as a fact. A big mistake many people make with this question type is to identify conclusion and evidence. There's no point in doing that.

What you should do is exactly this:

--treat every sentence as a fact, as true--no doubt about it
--see if you can combine the facts to arrive at a deduction
--aggressively scan for a choice that matches that deduction (you don't have to decide whether a choce is right or wrong the first time you read it--you just go through the choices aggressively looking for one that matches your prediction)
--select that choice, not care one whit about the other choices, and move on.

Let's do that here.

The first sentence is pretty easy to understand: All of Jon's friends say they know a heavy smoker who is, yet, healthy. But the second sentence tells us that John doesn't know anyone like that, and we also know it must be true that:



Therefore, despite what they "say", at least one of his friends is lying--that's it, that's our deduction.

Let's scan for the match..

Then, choice E clearly matches.

The passage does not establish that all of John's friends know the same life-long smoker. If we deny choice C--if John's friends didin't all know the same smoker--then no part of the passage is falsified. Thus, the passage has not proved that choice C must be true.