A massive pool of warm ocean water is causing changes in the atmosphere that could produce unusual weather around the world in t

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问题     A massive pool of warm ocean water is causing changes in the atmosphere that could produce unusual weather around the world in the next few months, the US National Weather Service reported on Monday.
    As a result of this phenomenon, known as El Nino, more rainfall than normal is likely this winter across some areas of the United States, with unusually warm or cold weather in other parts of the country.
    Currently the phenomenon is marked by a warm pool of water along the equator extending from the International Date Line nearly to the coast of South America. That water is nearly 4 degrees Fahrenheit above normal, explained Vernon Kousky of the climate center.
    This warm water "spreads almost a quarter of the way around the globe. So it’s massive and it has an impact on our weather. It has a "global influence... because it disturbs the atmosphere", said Dave Rodenhuis, director of the climate center.
    "El Nino is probably the most important climate event beyond the annual cycle of seasons", he added.
    Because the changes tended to be first noticed around Christmas, the phenomenon was given the name El Nino, which is Spanish for child, a term often used to refer to the baby Jesus.
    The phenomenon occurs every three to five years, sometimes in a mild form and sometimes strongly affecting weather patterns worldwide. Details of its cause are not fully understood, but when it occurs, unusually warm air can be pumped into Canada, Alaska and the northern United States. At the same time, conditions tend to be wetter than normal along the US Southeast Coast and the Gulf of Mexico. And the Atlantic and Caribbean hurricane season tends to be milder than usual.
    The strong El Nino of 1982-1983 was blamed for worsening the devastating drought in Africa, causing a series of severe winter storms to come ashore in California, spawning the first typhoon to strike French Polynesia in 75 years—followed by five more in five months—deluging Peru and Ecuador with torrential rains and promoting the worst drought in two centuries in Australia
    Overall damage was estimated at between $2 billion and $8 billion by a United Nations analysis and the death toll topped 1,500 worldwide.
What is least known about El Nino, according to the passage?

选项 A、The climate changes caused by El Nino.
B、The areas influenced by El Nino.
C、How often El Nino happens.
D、How El Nino is formed.

答案D

解析 原文第7段第2句开头的分句表明厄尔尼诺现象的成因仍不明了,因此选项D为本题答案。选项A在文章开头有讨论到,选项B在第7段有讨论到,在讨论这两点信息的时候,作者并未提及对它们了解甚少,因此这两个选项不是本题答案;第7段首句有提及这种现象每隔三到五年出现,故选项C也不正确。
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