1. Knock down sb. for a song 
   请某人唱只歌
2. Knock oneself out preparing for the experiment 
   努力为实验作好准备
3. Let's knock on for another half hour. 
   让我们再干半小时吧。
4. Run for the doctor and knock him up. 
   快去请医生，把他叫起来。
5. Knock a clock down for a trifling sum 
   以微不足道的价格拍卖掉了一只钟

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