如图所示，已知R₂=10Ω．开关断开时，电流表示数为0.6A；开关闭合时，电流表示数为0.9A．则电源电压为____ V，R₁=____ Ω．

C

    My name is Lin Tao. I am a Chinese boy. Now I'm studying in America. There are many kinds of clubs here. When I first come here, I don't know anything about clubs. I'm a shy boy, so I can't speak English well. In my first year in America, I don't like to speak to others. In my second year I have to change (改变) myself by doing some new things. I realize that school is not just about getting good grades (成绩), it's also about being part of a club. One day, when I go to the Media Center with my classmate Jim, he tells me something about the club. It's interesting, so I join it. I am in many clubs now. It's good to be part of a club. These clubs help me to make new friends.

放在水平地面上的一物块，受到方向不变的水平推力F的作用，F的大小与时间t的关系和物块速度v与时间t的关系如图所示。取重力加速度g＝10m/s²。由此两图线可以求得物块的质量m和物块与地面之间的动摩擦因数μ分别为（    ）