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, brain cells die quickly if what supply gets cut off?

pick from the following.
1). alveoli;
2). blood;
3). CO 2;
4). hydrogen;
5). Air;
6). Energy.;
7). electric pulses;
8). carbon dioxide;
Answer:
2).