单项选择题一般情况下，标定电极时所使用的标准缓冲溶液的pH值与待测溶液的pH值相差不能超过（）
单项选择题Questions 5 to 7 are based on the news report you have just heard. A. Shrinking potato farming. B. Heavy reliance on import. C. Widespread plant disease. D. Insufficient potato supply.
