Fact 1: Using a car causes pollution.

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

Given the two facts above, answer the question "What can be a major threat to health?" with the following options: 
- bicycling
 - using a car
 - smoking
 - skateboarding
 - viruses
 - Dehydration
 - influenza
 - doctors


 

  using a car       