What do unused roof racks do?

Answer: C — Waste fuel

Unused roof racks do not have any effect on a car's speed or fuel efficiency. In fact, having a roof rack attached to a car can actually decrease fuel efficiency due to added weight and wind resistance. While some people may use roof racks solely for aesthetic purposes, leaving them unused does not enhance the vehicle's appearance in any way. The only potential downside of leaving unused roof racks on a car is the waste of fuel that occurs from the added weight.