Q: What harms the respiratory and circulatory systems?
Options:
- (A) oil
- (B) rain
- (C) sulfur
- (D) cars
- (E) tires
- (F) coal
- (G) tar
- (H) bicycles
Let's think step by step below.

Air pollution harms the respiratory and circulatory systems. Cars are a significant source of air pollution. Cars harm the respiratory and circulatory systems.
Thus, the answer is (D).