我一直觉得这种用法不正确.因为记得只能是once, twice, 可是我在prep里遇到的题的提干里有at one time, 所以不懂怎么回事? 哪位高手解释下啊? prep2-88
Because Miranda, the smallest moon of Uranus, has a large number of different surface features, including craters, mountains, valleys, and fractures, some astronomers suggest that at one time repeated impacts broke the surface apart, and after which the fragments were subsequently rejoined because of mutual gravitational attraction.
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发表于 2007-8-4 15:59:00
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