Why Is It Particularly Difficult to Study the Mechanism of Action

Why is it particularly difficult to study the mechanism of action of inhalants?

A) Scientists do not understand their absorption and distribution in the body.
B) There is tremendous chemical diversity in this group of drugs, and it is likely that they may work in different ways.
C) These substances have arrived recently on the drug abuse scene and most researchers have not considered their dangers serious enough to merit study.
D) It is very difficult to design inhalant studies in animals, and the number of human users is too small to generate significant data.