Fact 1: Riding a bike does not cause pollution.

Fact 2: Air pollution Air pollution is a major threat to health.

Given the two facts above, answer the question "Riding a bike does not cause what?" with the following options: 
- Greenhouse gasses
 - forward motion
 - harmful substances
 - wear on tires
 - Greenhouse gases
 - major threat to health
 - contamination
 - sore bottom


 

      major threat to health   