Two tough questions

Hi, a friend asked me these two tough questions. I decide to post them in this forum instead of in Verbal and Math seperately, so that gurus can work on them rightaway... These are tough question so please take some time to figure out....
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In the past, most children who went sledding in the winter snow in Verland used wooden sleds with runners and steering bars. Ten years ago, smooth plastic sleds became popular; they go faster than wooden sleds but are harder to steer and slow. The concern that plastic sleds are more dangerous is clearly borne out by the fact that the number of children injured while sledding was much higher last winter than it was ten years ago.

Which of the following, if true in Verland, most seriously undermines the force of the evidence cited?

A. A few children still use traditional wooden sleds.

B. Very few children wear any kind of protective gear, such as helmets, while sledding.

C. Plastic sleds can be used in a much wider variety of snow conditions than wooden sleds can.

D. Most sledding injuries occur when a sled collides with a tree, a rock, or, another sled.

E. Because the traditional wooden sled can carry more than one rider, an accident involving a wooden sled can result in several children being injured.

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Math: Among 100 people, there are 48,45,and 57 read book a, b, c respectively. 20 people read all of the three books, 10 read nothing. What the number of people who read only two books?