题目

---Catherine, I have cleaned the room for you.---Thanks. You_____ it. I could manage it myself.       A. needn’t do      B. needn’t have done    C. mustn’t do     D. shouldn’t have done 答案：B与DNA相比，下列哪项是RNA特有的化学成分（）A.核糖和胸腺嘧啶B.核糖和尿嘧啶C.脱氧核糖和尿嘧啶D.脱氧核糖和胸腺嘧啶