Fact 1: Carbon monoxide is a gas produced by furnaces and other devices that burn fuel.

Fact 2: Excessive accumulation build-up of carbon monoxide can cause suffocation.

Given the two facts above, answer the question "What can gas furnaces cause?" with the following options: 
- matter vibrating
 - Pollution
 - rain
 - Stale air
 - condensation
 - emissions
 - suffocation
 - friction


 

       suffocation  