鉴别氮气和二氧化碳两种无色无气味的气体的最适宜的方法是（）

       Xiao Ming  is playing with his friend in front of a house. An old woman walks up to him. “My boy,” she asks., “ Is your mother at home?” “Yes ,” Xiao Ming says. The woman begins to ring（按） the door bell（门铃）, but there is no answer（应答）.
       She rings the door bell again. There is still no answer. The woman is not happy. She turns to Xiao Ming  and asks again, “Is your mother at home?” “Yes , she is.” Xiao Ming  answers. “But I ring the door bell twice and nobody（没人） comes to open the door,” the woman says.
“Oh, I'm sorry. This is not my house. My house is over there.”