Why is D correct?

Thanks for your question @Lucie D is correct because if those who have the most trouble falling asleep are those most likely take sleeping pills, then those in the sleeping pill group, on average, have a harder time sleeping than those in the behavior modification group. This is true before either group begins their respective treatments, so we cannot draw any conclusions from that fact that it remains true after both groups have begun their respective treatments. The two groups are not comparable, so if D is true then we cannot draw any conclusions about one type of treatment relative to the other.

I am trying to understand why D is the right answer, but the response is a little hard to follow. How do we know that these people have the hardest time falling asleep and how would that weaken it?