单选题
单选题如图，实线为一匀强电场的电场线，虚线为一带负电的粒子在该电场中运动的轨迹，P、Q为轨迹上的两点。若该粒子只受电场力作用，则关于匀强电场的方向和该粒子在P、Q两点的电势能WP、WQ的比较，正确的是（ ）。
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单选题设a，b，C为实数，且ab，则（ ）
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单选题Enough sleep is important to health.The amount of sleep（21）depends on the age of the per-son and the conditions in which sleep（22）.The young may need more sleep than the old,hut（23）eight hours are eno
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单选题我国历史上产生过许多关于山川地理的著作，北魏地理学家郦道元的著作是（ ）
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单选题The keyboard of a typewriter refers to the set of keys in it.Most typewriters haveqwertykey-boards.The name crones from the first six letters on the board′s second line.Theqwertyboard was set up by C.
单选题Enough sleep is important to health.The amount of sleep（21）depends on the age of the per-son and the conditions in which sleep（22）.The young may need more sleep than the old,hut（23）eight hours are eno
单选题碱性锌锰电池在生活中应用广泛，电池总反应可表示为：该电池工作时，负极上的反应物是（ ）。
单选题已知向量则χ=()
单选题Enough sleep is important to health.The amount of sleep（21）depends on the age of the per-son and the conditions in which sleep（22）.The young may need more sleep than the old,hut（23）eight hours are eno
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单选题以χ2-3χ-1=0的两个根的平方为根的一元二次方程是
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