库仑法测定水中COD时，若铂电极被沾污，可将电极放入2mol/L（）中浸洗片刻然后用重蒸馏水洗净。
A.草酸
B.醋酸
C.氨水
D.乙醇
参考答案：C
—Is your father a doctor?
—Yes, he is. He _______ in Town Hospital. [ ]
A. has worked
B. had worked
C. works
D. worked
（）的应用条件必须具备n>50，np或n（1-p）>5.
A.u检验
B.四格表X2
C.配对X2
D.行×列表X2
E.秩和检验
