在静电场中，下列说法中正确的是（ ）
A．闭合的电场线是不存在的
B．正电荷由静止释放，只受电场力作用时将沿电场线运动
C．负电荷由静止释放，只受电场力作用时将沿电场线运动
D．同一电荷在电场线较密的区域受的电场力较大
AD
电场线是假想的，从正电荷发出，终止于负电荷或无穷远，不封闭，故A正确；电场线在某点的切线表示该点的场强方向，即正电荷的受力方向，若电场线不是直线，其切线就不沿电场线，这样，正负电荷就不可能沿电场线运动，故BC错误；电场线密的地方代表电场强度大，故同一电荷在电场线较密的区域受的电场力较大，故D正确。
答案：AD。
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| Scientist Without Laboratories When you hear the word "scitentist", what do you think of? Many people think scientist are people in clean white coats who work in labs. And some scientists do work in labs. But there are 1 scientist who work in woods and jungles. These scientists are called "animal behavior (行为) scientists", and they study animals as they live in nature. Why don't these scientists catch animals and study them in zoos? The 2 is in the word"behavior". Animals behavior scientists want to learn 3 animal behave in their wild homes. When animals live in cages (笼子) or in zoos, they do not act the same as they do when they are 4 . They may fight 5 each other, or they may not eat, or they may not raise (抚养) their babies as they usually would. To see real animal behavior, scitentists must go where the animals 6 . So animal behavior scientists go into the jungles, the woods or the desert. There, their most important 7 are their eyes and ears. They watch and listen to the animals very 8 . They write down everything that happened in notebook. 9 they live near the animals, the scientists are careful not to frighten them. If the scientists are luncky, the animals will 10 no attention to them. Then the scientists can see how the animals really live. The behavior scientists hope what they've learned about animal behavoir can provide clues (提供线索) to help people learn to live together more happily. | ||||
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