题目内容：
You might think that "global warming" means nothing more than a rise in the world's tempera-ture. But rising sea levels caused by it have resulted in the first evacuation (撤离)of an island na-tion-the citizens of Tuvalu will have to leave their homeland. During the 20th century,sea level rose 8 -12 inches. As a result,Tuvalu has experienced low-land flooding of salt water which has polluted the country's drinking water.
Paani Laupepa, a Tuvaluan government official, reported to the Earth Policy Institute that thenation suffered an unusually high number of fierce storms in the past ten years. Many scientists con-nect higher surface water temperatures resulting from global warming to greater and more damagingstorms.
Laupepa expressed dissatisfaction with the United States for refusing to sign the Kyoto Protocol,an international agreement calling for industrialized nations to reduce their greenhouse gas emis-sioins,which are a main cause of global warming. "By refusing to sign the agreement, the US haseffectivly taken away the freedom of future generations of Tuvaluans to live where their forefathershave lived for thousands of years," Laupepa told the BBC.
Tuvalu has asked Australia and New Zealand to allow the gradual move of its people to bothcountries.
Tuvalu is not the only country that is vulnerable( 易受影响的 )to rising sea levels. MaumoonGayoon, president of the Maldives, told the United Nations that global warming has made his countryof 311,000 an "endangered nation".
根据以上内容,回答题.
（ ）The text is mainly about__________. A．rapid changes in earth's temperature
B．bad effects of global warming
C．moving of a country to a new place
D．reasons for lowland flooding
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答案解析：