—Do you know the boy under the tree?

—Yes, I know. He is creative and imaginative, but he is a little lazy _______. [ ]

A. once a time

B. at a time

C. at times

D. at time

电导率仪法测定电导率使用的标准溶液是（）溶液。测定电导率常数时，最好使用与水样电导率相近的标准溶液。

A．氯化钾 

B．氯酸钾 

C．碘酸钾