题干

如图甲所示, MN左侧有一垂直纸面向里的匀强磁场。现将一边长为l、质量为m、电阻为R的正方形金属线框置于该磁场中,使线框平面与磁场垂直,且bc边与磁场边界MN重合。当t=0时,对线框施加一水平拉力F,使线框由静止开始向右做匀加速直线运动;当t=t0时,线框的ad边与磁场边界MN重合。图乙为拉力F随时间变化的图线,不及摩擦阻力。由以上条件可知,磁场的磁感应强度B的大小为

A:

B:

C:

D:

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B

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The word that has changed the world

Apple is one of the most popular fruits in our life, and people eat apples every day. But thereare 1apples that are especially important to the history of our human beings and they have changed the whole world.
Long long ago when the world just started, apple was a forbidden  2in the Garden of Eden, however, a girl named Eve stole and ate it. Unfortunately, she was discovered by God, and was driven away 3 the Garden together with a boy called Adam. After they 4 the garden, Adam and Eve had their own babies and 5 the ancestors of all of us. This is the beginning of our world, history6morality.
In the Autumn of 1665, there 7 a young man sitting under an apple tree in his garden. His name was Newton. While he was thinking of one problem, suddenly, an apple dropped down from the tree and fell on his head.8 this, Newton discovered gravitation on that day and his new idea of science made people really 9 and happy, also he changed our opinions about nature and the way 10 we think, then people began to11what science was at that time.
And today, Jobs Steve, this crazy old man changed the world by his apple products. For example, iPhones12 be found 13 in the street, young people feel cool when they use this kind of new high-tech product. Don't you think14apple products have become a symbol of our time? As Jobs Steve often said he was born to change the world and he made it.15 he has died, people remember him and miss him, because he made our life more colorful and amazing.