chix475ntu did a great job trying to explain this one, but there still seems to be some confusion, so I thought I'd weigh in. The one thing I try to avoid when reviewing is writing off too many answer choices as "no effect" or "out of scope". This tends to ignore the subtlety of certain answer choices, and leads one to an attitude of ALL or NOTHING in terms of answer choices. Dangerous.
Conclusion: Plastic sleds are more dangerous than wooden sleds.
Premises: In past 10 years, plastic sleds have become more popular than wooden. More children injured on sleds now than 10 years ago.
Already, you should be able to see the assumption. We jumped to danger, without considering any other factors
Assumption: Some other factor is responsible for the jump in injuries (more people using sled, for example).
(A) A few children still use traditional wooden sleds.
PROBLEM: This doesn't weaken the argument. In fact, this is the argument. The whole idea is that lots of people are using plastic sleds, which are dangerous. A few children doesn't change that.
(B) Very few children wear any kind of protective gear, such as helmets, while sledding.
PROBLEM: This doesn't compare wooden sleds to plastic sleds, so doesn't explain why one might be more dangerous.
(C) Plastic sleds can be used in a much wider variety of snow conditions than wooden sleds can.
ANSWER: Now the plastic sled can get used a lot more often. This would explain why there are so many more injuries, because there's a lot more sledding going on. The argument that plastic sleds are more dangerous is now unlikely.
(D) Most sledding injuries occur when a sled collides with a tree, a rock, or another sled.
PROBLEM: Like B, this doesn't differentiate between wooden and plastic sleds.
(E) Because the traditional wooden sleds can carry more than one rider, an accident involving a wooden sled can result in several children being injured.
PROBLEM: We want to weaken the argument that plastic sleds are dangerous. Making wooden sleds hurt more people doesn't actually affect that. If anything, this strengthens the argument. If multiple people could get wounded on one wooden sled, then why would the #s be EVEN HIGHER for injuries on a plastic sled?
Hope that helps!
Tommy Wallach | Manhattan GMAT Instructor | San Francisco
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