—My car is really cheap at that price.  

—Yes, it is a real       , I must say.  [ ]

A. bargain

B. exchange

C. trade

D. business

在自然界的温度和压力条件下，哪种大气成份能在气态、液态和固态三者之间互相转化（即发生相变的唯一大气成份）？（）

A.氮气

B.氧气

C.水汽

D.二氧化碳