如图是由七个相同的小正方体堆砌而成的几何体，则这个几何体的俯视图是（　　）

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There are many famous museums in the world．The Louvre Museum is in Paris．It is a great art museum．The Palace Museum is well—known．And the British Museum，the Metropolitan Museum of Art are loved by people．People want to see all the beautiful things there．

如图所示，物体在拉力F的作用下沿水平面做匀速运动，发现当外力F与水平方向夹角为30°时，所需外力最小，由以上条件可知，外力F的最小值与重力的比值为（   ）