@KimJongUn, this question is a Cannot Be True question, so you're looking for the answer that can be 100% ruled out by the stimulus above. This doesn't mean that the other answers Must be true but only that they Could be true.
(B) says that all speed readers are of above-average intellects. There's nothing in the stimulus that says that this Must be true, but it could be. The conditions we have is that anyone who can fully-concentrate is of above-average intellect and that to successfully be trained in speed reading will likely increase your ability to concentrate. So it is possible that everyone who has ever trained in speed reading is of above-average intellect and capable of fully-concentrating. This isn't necessarily true, but it's possible, which means it's a wrong answer.
