Like racing cars that swerve and hit each other on a circular track, the moons' speed relative to each other is low because they travel in nearly the same orbit.

这有点像是突然转向的赛车在环形赛道上相撞，月亮们也在同一轨道公转，所以他们之间的相对速度很小。

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First is free flow in which the density of traffic is low enough to allow vehicles to travel at the maximum speed allowed.

首先是畅通无阻，交通密度足够低，使得车辆能以最高时速行驶。

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The train was invented in the early nineteenth century. It is fast in speed and low in price, especially suitable for the transportation of goods and long-distance passenger travel.

火车诞生于十九世纪初，其特点是快速，廉价， 特别适合陆上货物运输以及旅客长途旅行。

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