连词成句。

1. knew, a, special, he, baby, they, was (.)

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2. the, is, in, sky, star, shining, the (.)

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3. trees, on, we, stars, Christmas, put, our (.)

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平面汇交力系平衡的充分必要（解析）条件是：力系中各力在X、Y坐标轴上的投影的代数和都等于零。（）