下列哪一项正确（） A.健康危险因素评价的应用在于开辟定性研究健康危险因素的途径。

     One day a rich man met Sam. The rich man asked,"I hear you're very clever and nothing is difficult to
you. Can you tell me why you are so clever?" 
     Sam answered with a smile,"Oh, I am not clever. Instead, you are too foolish." The rich man became
very angry.
     Sam said,"Please don't be angry. If you don't believe me,now let me ask you a question. If you put
together one group of sheep,how many groups of sheep do you have?"
     "Why! That's an easy question. One and one is two, anybody knows that. I have two groups of sheep." 
     Sam laughed and said,"you are wrong. When two groups of sheep is put together, there is still one group.
That's the easiest question in the world."