题干

如图所示,质量为m的木块P在质量为M的长木板ab上滑行,长木板放在水平地面上一直处于静止状态.若长木板ab与地面间的动摩擦因数为μ1,木块P与长木板ab间的动摩擦因数为μ2,则长木板ab受到地面的摩擦力大小为                                                       (  )

A:μ1Mg

B:μ1(mM)g

C:μ2mg

D:μ1Mgμ2mg

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C

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       As I rushed among the tables, suddenly, a  1  asked if I could mind a child. I was quite   2 but I could tell the man was quite desperate. So I had to make a  3  with him that if he wanted his son to stay and wait for him, he had to 4  something in the shop.

       This was quite  5  for the man to do; I could  6  he was quite poor by the  7  he was dressed. It looked as if he had tried to come in his 8 clothes. But they still looked a bit old and worn out 9 he had often worn them, just to make himself look best. Looking down I could see his 10  were also a bit torn and the heels were in a terrible state. I thought he was going for a job interview. In the end he bought a small box of 11 for his little son and seated him down in the corner. I could tell the boy was feeling down and only could   12  with a bit of cheering up.

       Seeing that the kid sat there for quite a while, I was a bit  13  as the poor little kid was sitting  14  his own. People watched him and some kids even came over to pick on him. I was15  to see one of them even knocked his cookies off the table.He quickly went and got them back without saying a word.I was hoping his father would 16  up and come back for his son's 17  .

        At closing time, the only person left was the little boy.After a while, the father finally came in with a tiresome look. I  18 he didn't get any job.As they were leaving, I offered the kid a little cake, but the man rejected it. I19  he felt as if I had just abused him.Of course I didn't mean to.But I could understand why he felt like that.I only wished them good  20  whatever happened.

The man opened the door to leave-father and son hand in hand.