How do you like to go to school like one in Oxford, England ? There are no r_________ classes. The students go from one group to a _________ when they want to. You may find students of fourteen, sixteen or twenty-five years old all in the s_________ group. They work at their o________ studies. Nobody tells them what they should or shouldn’t be doing. The day I visited, school b_________ at nine. Some s_________ were working at a tape recorder and listening to their lessons. Others were watching TV on Maths. A group was reading in the school l________. I didn’t see anybody just sit doing n__________. Everybody was studying. At lunch time I could also see students working. They were talking a________ their lessons while they were e_________. When you think of these students you can be sure it is one of the best schools in the world.

对水源水、出厂水和部分有代表性的管网末端水，至少（）进行一次常规检验项目的全分析。

A.每一年

B.每半年

C.每季度

D.每月