The ocean bottom, a region nearly 2．5 times greater than the total land area of the Earth, is even today largely unexplored．Until about a century ago, the deep-ocean floor was completely inaccessible and hidden beneath waters averaging over 3,600 meters deep．Totally without light and subjected to intense pressures hundreds of times greater than at the Earth’s surface, the deep-ocean bottom is a strangeenvironment to humans, in some ways as forbidding and remote as the outer space．
Although researchers have taken samples of deep-ocean rocks for over a century, the first detailed global study of the ocean bottom did not actually start until 1969, with the beginning of the National Science Foundation’s Deep Sea Drilling Project(DSDP)．Using techniques first developed for the offshore oil and gas industry, the DSDP’ s drill ship, the Glomar Challenger, was able to maintain a steady position on the ocean’s surface and drill in very deep waters, taking samples of rock from the ocean floor．
The Glomar Challenger completed 96 voyages in a 15-year research program that ended in November 1983．During this time, it sailed 600,000 kilometers and took almost 20,000 samples of rocks aroun d the world．Those samples have allowed geologists to reconstruct what the planet looked like hundreds of millions of years ago and to make out what it will probably look like millions of years in the fu ture, Today, largely on the strength of evidence gathered during the Glomar Challenger’ s voyages, nearly all earth scientists agree on the theories of plate tectonics （构造学）and continental drift that explain many of the geological processes．
The samples of rocks drilled by the Glomar Challenger have also provided a climatic record stretching back hundreds of millions of years．The information of past climatic change can be used to predict future climates．
68．What does the underlined word “inaccessible” in line 3 means?
A．unrecognizable B．unreachable C．unusable D．unreasonable
69．Why does the author mention “outer space” in the first paragraph?
A．The Earth’s climate millions of years ago was similar to that in outer space．
B．It is similar to the ocean floor in being strange to the humans．
C．Rock formations in outer space are similar to those found on the ocean floor
D．Techniques used by scientists to explore outer space were similar to those used in ocean exploration
70．Which of the following is true of the Glomar Challenger?
A．It is a type of submarine． B．It is an ongoing project．
C．It has gone on over 100 voyages． D．It made its first DSDP voyage in 1968．
71．Which of the following is NOT mentioned in the passage as being a result of the Deep sea Drilling Project?
A．Geologists were able to determine the Earth’s appearance millions of years ago．
B．Two geological theories became more widely accepted by scientists．
C．Geologists observed forms of life never before seen
D．Information was revealed about the Earth’s past climatic changes．
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， 
，a点的磁感应强度大小为
；
，单位长度受到的安培力的大小为F=BIL=