单选题 Why are RDAs regularly updated?

A、 New discoveries in the science of nutrition are constantly being made.
B、 Americans' diets are constantly changing.
C、 As people age, their nutritional needs change.
D、 Very little is currently known about nutrition.

【正确答案】 A

【答案解析】[分析] 细节题型。从文章第二句(These are known as the Recommended Dietary Allowances(RDAs) and are updated regularly based on new research in nutrition．)得知RDAs会根据营养学上的新发现经常更新推荐的日摄食量。所以“推荐的日摄食量经常更新的原因是营养学上经常有新的发现”，选项A正确。