题目内容：
Explorer of the Extreme Deep Oceans cover more than two-thirds of our planet．Yet，just a small fraction of the underwater world has been explored．Now，Scientists at the Woods Hole Oceanographic Institution（WHOI）in Massachusetts are building an underwater vehicle that will carry explorers as deep as 6，500 meters（2l，320 feet）．The new machine，known as a manned submersible or human-operated vehicle（HOV），will replace another one named Alvin which has an amazing record of discovery，playing a key role in various important and famous undersea expeditions．Alvin has been operating for 40 years but can go down only 4，500 meters（14，784 feet）．It’s about time for an upgrade，WHOI researchers say.Alvin was launched in 1 964．Since then，Alvin has worked between 200 and 250 days a year，says Daniel Fornari，a marine geologist and director of the Deep Ocean Exploration Institute at WHOI．During its lifetime，Alvin has carried some 12，000 people on a total of more than 3，000 dives．
A newer，better versions of Alvin is bound to reveal even more surprises about a world that is still full of mysteries，Fornari says．It might also make the job of exploration a little easier．“We take so much for granted on land，”Fornari says．“We can walk around and see with our eyes how big things are．We can see colors，special arrangements．”
Size-wise．the new HOV will be similar to Alvin．It'll be about 37 feet long．The setting area inside will be a small sphere，about 8 feet wide，like Alvin，it'll carry a pilot and two passengers．It will be just as maneuverable．In most other ways，it will give passengers more opportunities to enjoy the view，for one thing．Alvin has only three windows，the new vehicle will have five，with more over-lap so that the passengers and the pilot can see the same thing．
Alvin can go up and down at a rate of 30 meters every second，and its maximum speed is 2 knots（about 2．3 miles per hour），while the new vehicle will be able to ascend and descend at 44 meters per second．It'll reach speeds of 3 knots，or 3．5 miles per hour．What is Alvin? A．A research institut
B．A transporting vehicl
C．A submersibl
D．A scientis
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