如图所示，置于水平地面上的立柱质量为M，有一质量为m的猴子从立柱上下滑．巳知猴子沿立柱滑下时，受到摩擦力为f，则猴子下滑时立柱对地面的压力为（　　）

完形填空

    Nancy is a funny girl. Her 1  is full of fantastic ideas. Like any other child, she has 2  in her life, but she always solves it in funny ways.

    Nancy's classmate Lionel lost a tooth at school last month. The teacher 3  him a cute tooth necklace (项链). Nancy found it was nice and she wanted one too.

    One morning Nancy found one of her teeth was loose (松动的). "I will get a nice tooth necklace 4  the tooth falls out at school," she thought. Nancy did everything that she could

    5to make sure she lost her tooth at school. However, things didn't always happen as she 6 . The tooth finally 7 out on her way to school. Nancy told her teacher what had happened. She was 8 . She didn't think she could get the special necklace.

    To 9 surprise, the teacher decided to give Nancy the necklace. "Your tooth dropped out close to the school, didn't it?" the teacher said. "And you're really an honest (诚实的) girl." Finally, Nancy's dream came 10  .

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        沧州南一寺临河干，山门圮于河，二石兽并沉焉。 阅十余岁，僧募金重修，求二石兽于水中，竟不可得，以为顺流下矣。棹数小舟，曳铁钯，寻十余里无迹。一讲学家设帐寺中，闻之笑曰：“尔辈不能究物理。是非木杮，岂能为暴涨携之去？乃石性坚重，沙性松浮，湮于沙下，渐沉渐深耳。沿河求之，不亦颠乎？”众服为确论。一老河兵闻之，又笑曰：“凡河中失石，当求之于上流。盖石性坚重沙性松浮水不能冲石其反击之力必于石下迎水处啮沙为坎穴。渐激渐深，至石之半，石必倒掷坎穴中。如是再啮，石又再转。转转不已，遂反溯流逆上矣。求之下流，固颠；求之地中，不更颠乎？”如其言，果得于数里外。然则天下之事，但知其一，不知其二者多矣，可据理臆断欤？