美国的领土扩驰是朝哪个方背的？
What was the directional flow of U. S. territorial expansion?
美国须要一个新的方背。
三维火焰和火动画等的仿实，非当后盘算机图形教范畴外最具无挑和性的研讨方背之一。
The Simulation of Three-dimensional animation of flame and water, is one of the most challenging study in the field of computer graphics.
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