Fact 1: Brain cells die quickly if their oxygen supply is cut off.

Fact 2: Brain atrophy means the brain matter has shrunk, often because brain cells are dying or dead.

Given the two facts above, answer the question "what causes brain atrophy?" with the following options: 
- Smoking
 - sodium
 - Dehydration
 - sweating
 - death
 - birth
 - Veins and arteries.
 - lack of oxygen


 

        lack of oxygen 