单选题 Mr. Black doesn't think that all bad practices should be allowed. In his opinion, there are______that should be kept up.

【正确答案】 C
【答案解析】[解析] Mr.Black认为并非所有不标准的惯例都可以保留。哪些可用，哪些不可用，应有一定的标准，所以答案为C。