The average life expectancy of people who drink alcohol heavily is lower than that of people who do not drink heavily, and people who smoke tobacco have lower life expectancies on average than those who refrain from smoking. Yet the average life expectancy of people who both drink heavily and smoke tends to decrease when such people give up drinking. Which one of the following most helps to reconcile the discrepancy described above? ['Most heavy drinkers who are also smokers tend to smoke more heavily if they stop drinking.', 'The life expectancy of smokers who stop smoking increases, even if they have smoked for many years.', 'Most heavy smokers who are not heavy drinkers tend to gain weight if they stop smoking.', 'The average life expectancy of relatively light smokers is lower than that of people who have never smoked at all.']