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

Fact 2: Blood carries oxygen to the brain.

Given the two facts above, answer the question "brain cells die quickly if what supply gets cut off?" with the following options: 
- alveoli
 - blood
 - CO 2
 - hydrogen
 - Air
 - Energy.
 - electric pulses
 - carbon dioxide


 

  blood       