工作中曾经存在大量问题。
NASA火星探测器的火星矿物研究显示火星上曾经有水流动，但是这些水含有大量的盐和酸。这就增加了火星是否曾经支持生命存在的疑虑。
Minerals on Mars studied by the NASA rovers suggest water once flowed on the planet's surface, but was very salty and acidic, raising doubts about whether it could have supported life.
研究者旨在希望发现一种名为“大众”的蟋蟀仍然存在于城市的某些地区，这种蟋蟀曾经在纽约大量存在，现在却很少见了。
The researchers are hoping to find evidence that the Common True Katydid, once plentiful in New York City but now rare, is still thriving in some regions of the city.
大量的二氧化碳曾经使火星的气候变得温和，并且目前在火星上仍然可能存在着二氧化碳。 在冻结的泥土和极地冰盖中还可能存在着水。
Much of the carbon dioxidethat once warmed Mars is probably still there, in frozen dirt and polar icecaps, and so is the water.
土族民族狩猎野牛，这是一种曾经在美洲平原上大量存在过的一种野生动物。
They hunted bison, a type of wild animals which used to exist in huge numbers on the plains of America.
土族民族狩猎野牛，这是一种曾经在美洲平原上大量存在过的一种野生动物。
They hunted bison, a type of wild animals which used to exist in huge numbers on the plains of America.
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