________（50）A. More water was added to the planet
________（50）
A. More water was added to the planet during several hypothesized large impacts from asteroids from the outer asteroid belt.
B.We know that the oceans existed as early as 100 million years after the formation of the Earth.
C.Still,the oceans only make up 0.023%of the Earth’s total mass.
D.Astronomers say Earth s water may have come from space.
E.The snow line is located in the outerasteroid belt,between the orbits of Mars and Jupiter.
F. Many geologists believe that the majority of Earth’s water generated through this process.
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答案：F
文中的最后一段主要是讲地球上水的来源。前一句：In the early seas of Earth,hydrogen sulfide would have been in great supply,which,when reacted with carbon dioxide from the atmosphere during photosynthesis in sulfide-reducing bacteria,would have produced hydrogen , sulfur , and water．在地球早期的海洋中，硫氢化合物非常丰富，在去硫细菌的光合作用的影响下，它与大气层中的二氧化碳产生化学反应，产生大量的氢、硫，以及水。这是地球自身产生的水，而F选项Many geologists believe that the majority of Earth’s water generated through this process(许多地理学家认为地球上水的产生主要通过这一进程实现）正好承接前一句，所以F选项正确。