OK. I was a little nervous about doing so b/c of possible copyright restrictions or whatever. But here goes:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument
(C) fails to take into account what proportion of the population have used cocaine
As I stated, I don't dispute that this C is the right answer choice - i got it right through POE. But I honestly can't see what's wrong with the argument. Given the premises (5/100 non-cocaine users test +; 99/100 cocaine users test +) I don't see how to construct a hypothetical scenario that meets these premises but not the conclusion of the argument.
Thanks for any help.