下列计算正确的是（   ）

做匀速圆周运动的物体，受合力为，轨道半径为，周期为，则物体在运动一周的过程中

阅读理解Man or woman or a girl ?
One day, there is a man with long hair in the park. He is sitting on a chair, at this time an old man asks him, Is it a boy or a girl over there? She has long hair. I think she is a girl. Can you tell me?”
    “Yes. You are right. She is my daughter.”
    “Oh, I know. You must be the girl's mother.”
    “No， I'm her father.”The man is very angry（生气）.

以下是明明同学对串联电路电压规律进行的探究实验．

【猜想与假设】串联电路总电压等于各用电器两端的电压之和

（1）按图所示的电路图连接电路 ​

（2）闭合开关，用电压表测出L₁两端的电压；在测L₂两端的电压时，明明同学为了节省实验时间，采用以下方法：电压表所接的B接点不动，只断开A接点，并改接到C接点上；

（3）测出AC间的电压．

耕地施用有机肥后，土壤要进行适当的疏松，才有利于提高土壤中可被植物吸收的N素的含量。原因是(    )。