单选题
The concentration of a certain chemical in a full water tank depends on the depth of the water. At a depth that is x feet below the top of the tank, the concentration is 3
【正确答案】
E
【答案解析】[解析] “to the nearest”是中文中用四舍五入的方法取近似值的意思。
由题意可得：
[*]
由题意可得：
[*]