对电位滴定仪的电极维护应包含的内容是（）。
A.电极洗涤
B.电极保存
C.电极更换
D.电极选择
参考答案：A, B
△ABC的三边为a，b，c且（a+b）（a-b）=c2，则（ ）
A．a边的对角是直角
B．b边的对角是直角
C．c边的对角是直角
D．△ABC不是直角三角形
After a day's exhausting climb we arrived at ________ we had been told was Garden in the Air.
A．what
B．which
C．that
D．where
