第一和第三次的爆发的火山灰覆盖了大部分北美州。
Both the first and third eruptions covered most of North America with ash.
我们已经根除了三种野生脊灰病毒菌株之一。
We have already eradicated one of the three strains of wild polio virus.
鲨鱼的种类有很多，而经常被载攻击人的只有四种 – 牛鲨（学名白真鲨）、虎鲨（学名居氏鼬鲨）、大白鲨（学名噬人鲨）、白鳍鲨（学名灰三齿鲨）
While there are numerous types of sharks, only four are most frequently cited in encounters with humans—the bull shark, the tiger shark, the great white shark and the oceanic white tip shark.
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