Studies Testing How Readily Rats with Self-Administer Toluene Demonstrated That

Studies testing how readily rats with self-administer toluene demonstrated that rats adjusted their response rates to obtain

A) variable toluene concentrations, thereby demonstrating the unpredictable effects of the drug.
B) consistent peak toluene concentrations, thereby demonstrating tolerance to the drug.
C) consistent low toluene concentrations, thereby demonstrating an absence of reinforcing effects of the drug.
D) moderate toluene concentrations, thereby demonstrating the weak reinforcing effects of the drug.