(C) says, "The longer a car trip is, the more likely a passenger is to help the driver maintain alertness."
We're looking to resolve the paradox in the stimulus. If passengers help drivers to remain alert, then it makes sense that they'd be helpful to drivers on longer drives because they can keep them awake and focused on the road. (C) shows how having a passenger can benefit a driver on a long trip, so it helps to explain the paradox and is the correct answer choice.
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