"Mister, why are cars fast?"
Translation:아저씨, 자동차는 왜 빠릅니까?
차 can be an abbreviation of 자동차 and mean "car", but 들 is not used in general statements. So if you use "차들이" that would be "Why are the cars fast?" (as in some specific cars you are pointing to), rather than asking why cars in general are fast (which is what this exercise is looking for).