a confusing CR

老虎油

Hi Hexin，

In my opinion，the key to the question should be （C）。

Pay attention to the premise of the text‘s conclusion：“when a randomly chosen group of people is tested ”，but the premises of  the text’s evidence are 5 out of 100 persons，who definately do not use cocaine， and 99 out of 100 people ， who are definately cocaine users。Do you find out the difference now？Good luck！

hexin

Thanks very much for your explaination.  You are correct, now I see.

老虎油

You are more than welcome! Good luck!

晴天猪

不好意思，小猪来晚了~~
谢谢老虎的解答，同意你的观点
这道题的关键在于"randomly chosen"

flora_wang

还是有点迷惑
能有人再解释一下么....