You are presented with the question "Riding a bike does not cause what?" and the following answer choices: 
- Greenhouse gasses
 - forward motion
 - harmful substances
 - wear on tires
 - Greenhouse gases
 - major threat to health
 - contamination
 - sore bottom

Now knowing that riding a bike does not cause pollution and air pollution Air pollution is a major threat to health, choose the best answer.


 

      major threat to health   