the answer is a. skin color
Drugs that break down a psychological barrier against doing other drugs are known as gateways drugs.
A very known gateway drug (or at least one that has been considered as such) is marijuana or weed. A drug that is being widely consumed in various countries and is also being sold as a legal drug in some countries.
That something is wrong with them and they are not doing the best since their oxygen level is low
A and B probably take the longest