What is the likely effect on heart rate when a subject is under the influence of both marijuana and cocaine?

The correct answer is that the heart rate will be elevated when a subject is under the influence of both marijuana and cocaine. Cocaine is a stimulant that activates the central nervous system, leading to an increase in heart rate and blood pressure. Its primary action is to enhance the effects of neurotransmitters like norepinephrine, which directly stimulates the heart and increases cardiac output.

While marijuana has more complex effects and can cause both an increase and a potential decrease in heart rate depending on the dose and individual response, the stimulant nature of cocaine typically dominates, especially in cases where both substances are present. This combination can lead to a synergy where the stimulant effects of cocaine are intensified by marijuana's influence, resulting in an overall elevation of heart rate.

Thus, when assessing the combined effects of both drugs, the likelihood of an elevated heart rate is strong, making this the correct choice.