pg 143 #22 (PT48 S4 #25) question

Hey Dave,
This problem is giving me a hard time - this is what I have so far:

In real life, there could be much more than 60 psychological problems, say 500 or more. Also, the sample being large and diverse is not enough to prove that they represent everybody in the world -- the sample could have been as many as a 1000 of people, but we can’t conclude anything about MOST people just from a sample of 1000 people. So behavioral therapy clearing up 75% of 60 problems for a large sample of people alone cannot prove that that is all that MOST people need. The only valid conclusion we can make here is that for SOME people, 50 weeks of behavioral therapy is enough to eradicate their psychological problems.

Am I on the right track..?

The way answer choice (B) is worded confuses me and I don't get how it's the flaw of the argument. I'm also not sure why (E) is incorrect.. Some explanation would be much appreciated! Thanks!