The chemical in cigarettes that prevents oxygen from getting into the bloodstream is:
a. tar
b. nicotine
c. carbon monoxide
d. all of the above

A. Tar

Explanation:

In cigarettes, they were originally created with paper and tobacco but later in around the 50s they added tar to make the effects of the tobacco last longer.  But the side effect of tar is far worse than without putting it in the cigarette in the first place.  The tar coats the inside of your lungs which holds in everything else, this also prevents oxygen from going in and out through your breathing tube and prevents oxygen from going into the blood stream.  Another thing we should note is that tar will also increase the chance of lung cancer, though nicotine will too, and smoke, this is why smoking is bad.