参考答案:条件是:力系中所有各力在两个相互垂直的坐标轴上投影的代数和都等于零;力系中所有各力对平面内任意点的力矩的代数和等于零。
Your math instructor would have been happy to give you a makeup examination had you gone and explained that your parents had been ill at the time.
A. would have been
B. had you gone
C. explained
D. had been
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