According to Conditioning Models of Drug Tolerance,why Do Drug Users

Multiple Choice

According to conditioning models of drug tolerance,why do drug users start using larger and larger doses over time

A)  The drug effect is a UCR that is very rewarding, so more is used over time. 
B)  The CR compensates for the effect of the drug, resulting in a smaller high than usual with the same dose. 
C)  The CS becomes ineffective in eliciting the typical high, so a more salient CS must be used. 
D)  The CS becomes larger and larger over time, so the drug dose must become larger as well.