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【单选题】
关于功的概念有以下几种说法: (1) 保守力作正功时,系统内相应的势能增加。 (2) 质点运动闭合路径,保守力对质点作的功为零。 (3) 作用力和反作用力大小相等、方向相反,所以两者所作的功的代数和必然为零。 在上述说法中
A.
1、2是正确的
B.
2、3是正确的
C.
只有2是正确的
D.
只有3是正确的
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