问题
单项选择题
One implication in this passage is that ______.
A．all RDAs have been established
B．not all RDAs have been established yet
C．it's not important to know RDAs
D．RDAs are necessary only for sick people
答案
参考答案：B
解析：[分析] 推断题型。 见文章第二句：These are known as the Recommended Dietary Allowances(RDAs) and are updated regularly based on new research in nutrition．可知RDAs要经常更新，所以选项A不对；选项C、D也被排除；只有选项B正确。