The neurotransmitter has been a primary focus of drug abuse research because of its well-known effect on a part of the brain's reward system known as the nucleus accumbens. drugs that increase the activity of this neurotransmitter are most likely to result in physical dependence.

The correct answer is:  "dopamine" .

            "The neurotransmitter    dopamine  has been a primary focus of drug abuse research because of its well-known effect on a part of the brain's reward system known as the nucleus accumbens.  Drugs that increase the activity of this neurotransmitter are most likely to result in physical dependence."

This is a pretty on point question because all we need to do is replace 's' in the equation with '35'.the calculation will be9.95 + 0.30s = c9.95 + 0.30 (35) = c9.95 + 10.5  = cc = 20.45