The possessive pronoun "their" in the last paragraph is used in place of ______.

A．In rich countries people are becoming more aware of the health risks of cigarettes and smokers are starting to quit.
B．More than half of the smokers in the world are in Asia.
C．More and more people in poorer countries are getting into the habit of smoking.
D．Despite years' cigarette advertising restrictions in rich countries, cigarette sales worldwide are ever increasing.

A．they are afraid that health-care costs will rise greatly due to smoking-related diseases
B．they are worried that there is no cure for lung-cancer and emphysema
C．they are worried that too much money will be spent on advertising
D．they are afraid that children may take up the habit of smoking when they are too young