碘量法测定水中游离氯、总氯和二氧化氯中,若发现硫代硫酸钠溶液在使用一时期后变浊或有硫的沉淀析出,应如何处理?
参考答案:
应根据情况将溶液过滤后重新标定,或重新配制。
Mrs. Smith can _____________ piano.[ ]
A. play the
B. played
C. plays the
.--Your parents were anxious about you. You _____ home without saying a word.
-- It’s my fault.
A.mustn’t be
B.shouldn’t have left
C.needn’t have left
D.couldn’t have left