What factor might be important in maintaining smoking during the day when dopamine receptors are most likely already desensitized in smokers?

a. Smokers can avoid nicotine withdrawal symptoms

b. Sensory aspects of smoking act as secondary reinforcers of smoking through many past pairings with nicotine

c. Smoking causes a reduction in monoamine oxidase activity, slowing dopamine metabolism

d. The cigarette smoke actually has pleasant qualities for the regular smoker, although it remains aversive for the nonsmoker

e. All of the above