单项选择题A．Dr Jarvik s studies on mental functioning of the twins.B．The explanation of the causes of the mental decline.C．A person s mind decline being irrelevant to his aging.D．The difference between middle-aged and older people.

单项选择题A．The fact that the mother gradually loses her land.B．The fact that the father has got his own house and land.C．The changes in economic relations.D．The changes in family relationships.