We must keep the classroom clean for dirt and disease go ________．A.from time to timeB.hand in handC.one after anotherD.step by step
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B解析我们必须保持教室清洁因为肮脏和疾病往往是并行的。from time to time：偶尔；hand in hand：手拉手；关系密切的；并行；one after another：一个接一个地；step by step：逐步地