高血压的预防应从何时开始（）

A.儿童少年期

B.婴儿期

C.青年期

D.中年期

E.老年期

Which of the following conditions exists when the economic gain to society is maximized（）

A. The sum of consumer and producer surpluses for a good or service is maximized. 

B. The difference between consumer and producer surpluses for a good or service is maximized. 

C. The price and quantity for a good or service is such that consumer and producer surpluses are equal.