(B) says, "Of those members of the population who do not watch television, the percentage who read a newspaper every day has also shown a dramatic decrease."
We're looking to wreck the explanation in the stimulus. With (B), we know that newspaper reading has decreased even among the subset of the population that doesn't watch television. This shows us the purported effect (a decrease in newspaper reading) without the purported cause (increased television viewing), so this damages the explanation given in the stimulus and is the correct answer.
Does this make sense? Let us know if you have any other questions!
