Wagon trains — groups of wagons — rode in four lines across the land when they passed through dangerous country. If attacked, the wagons could quickly form a circle for defense.

马车队(即一群马车)，当他们经过危险区时，他们排成四排向前行进，如果遭受到攻击时，这些马车就会很快地形成一个保护圈进行抵抗。

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The key problem of stability analysis of roadbed slope is how to determine the most dangerous sliding circle and calculate the minimum safety factor, this is a nonlinear problem.

路基边坡稳定性评价的关键是如何确定最危险滑裂面的位置并计算与之相对应的安全系数，是非线性优化问题。

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While it used to be dangerous to walk a few blocks to a nightclub in Logan Circle, that neighborhood is now known for its sprawling Whole Foods grocery store.

WSJ: Washington, D.C.: The New Boomtown