题目

 －You look a bit worried. Do you need any help? －Yes. The task is ______ I could do myself. A. less than         B. more than        C. no less than         D. no more than 答案：B9．袋中装有10个红球、5个黑球．每次随机抽取1个球后，若取得黑球则另换1个红球放回袋中，直到取到红球为止．若抽取的次数为ξ，则表示“放回5个红球”事件的是（　　）A．ξ=4B．ξ=5C．ξ=6D．ξ≤5