老师安排座位的时候，把我和一个不认识的同学安排在一起，不知道我能不能和他成为学习上的好伙伴，这时候，我应当（   ）

用手平握一直立酱油瓶静止悬在空中，如图所示，下列关于摩擦力的说法中正确的是（   ）

下列有关物质的性质或用途的叙述中，正确的是（　　）

       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.”