Marjorie Takes an Anti-Anxiety Drug So That She Can Sleep

Marjorie takes an anti-anxiety drug so that she can sleep better and remain calm enough to study effectively.She begins by taking 25 milligrams every evening,but within a month,this dose is no longer helping,so she takes two pills.After another month,this dosage is no longer effective.This is happening because

A) her immune system is rejecting the drug.
B) she has developed tolerance,which means that her liver can no longer metabolize the drug,so it remains active for too long.
C) the number of receptors to which the drug binds on neurons has declined.
D) the number of receptors to which the drug binds on neurons has increaseD.