题干

如图,A为螺线管,B为铁芯,C为套在铁芯B上的绝缘磁环,现将A、B、C放置在天平的左盘上,当A中通有电流I时,C悬停在A的上方,天平保持平衡;当调节滑动变阻器,使A中的电流增大时,绝缘磁环C将向上运动,在绝缘磁环C上升到最高点的过程中,若不考虑摩擦及空气阻力的影响,下列说法中正确的是(   )

A:天平仍然保持平衡

B:天平左盘先下降后上升

C:天平左盘先上升后下降

D:绝缘磁环速度最大时,加速度也最大

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B

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    The Republic of Vanuatu is an island nation located in the South Pacific Ocean. It was one of the poorest nations in the region. Within the last five years, however, it has become a hot spot for adventure travelers. In 2006 it was voted the happiest place on Earth. The reason is not that Vanuatu are the richest but that Vanuatu has white-sand islands, clear waters, the world's most accessible live volcano, great diving and food that grows faster than it can be picked.

    With a population of about 221,000, Vanuatu has 113 languages, which is a result of 3,000 years of immigration from many Pacific countries. There are three official languages: English, French, and Bislama. Bislama developed from South Pacific English, a simplified language that traders and other foreigners used it to communicate with speakers of the many local languages.

    Most of the islands are mountainous and of volcanic origin. There are several active volcanoes in Vanuatu, including Yasur on the island of Tanna, one of the world's most accessible volcanoes. If you've ever fancied getting close to a volcano, here's your chance.

    Vanuatu Post's Underwater Post Office has quickly become one of the busiest post offices for postcards in the world! Visitors from around the world want to experience the world's first under- water post office. The Post Office is only 50 meters offshore and at just three meters below the surface, very appealing to the young and young at heart. Since opening in 2003, the underwater post office has received about 100,000 visitors. They have posted one of the waterproof(防水的)postcards available in Vanuatu; these cards are collected regularly by one of Vanuatu Post's four trained divers. The underwater branch is manned daily for usually about an hour, longer with demand.