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【单选题】
对于具有理想约束的质点系，其平衡的充分必要条件是（ ）。A.
作用于质点系的所有约束力所作的虚功等于零。
B.
作用于质点系的所有主动做的功等于零。
C.
作用于质点系的所有主动力在任何虚位移中所做的虚功的和等于零。
D.
作用于质点系的所有主动力在任意一个虚位移中所做的虚功等于零。
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