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问题 选择题
如图所示，AB是一质量为m的均匀细直杆，A端靠在光滑的竖直墙壁上，B端置于水平地面上，杆身与竖直方向夹角为θ，杆与地面的摩擦系数为μ，保持平衡，则此时杆与地面的摩擦力为（　　）

A．mgtgθ

B．mgsinθ

C．μmg

D．μmg

答案

答案：A

选择题

四幅图中正确表示北半球夏至日的是   （   ）
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补全对话，情景问答
根据表格中的信息完成对话。

Name: Yao Ming
Date of birth: 12/09/1980
1989: Started playing basketball
1994: Joined the Shanghai basketball team
1997:  Joined the national team
2002: Joined NBA

A: __1__
B: My favorite basketball player is Yao Ming.
A: __2__ 
B: He was born in 1980.
A: __3__ 
B: He started playing basketball when he was nine.
A: __4__ 
B: He joined the Shanghai basketball team in 1994.
A: When did he join the national basketball team?
B: In 1997.
A: __5__ 
B: He joined NBA in 2002 and he has made great achievements (成就).
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