(A) Exactly
(B) Clearly
(C) Economically
(D) Practically
48. According to the passage, computers are used on the Glomar Challenger in order to
(A) measure the spread of the ocean floors
(B) lower its drilling pipes into the water
(C) keep it in one place
(D) detect the location ofthe Earth’s mantle
49. The Phrase "stable platform" in Line 11 refers to
(A) the Glomar Challenger
(B) a ship’s anchor
(C) sound-wave guiding systems
(D) computer-controlled propellers
50. For which of the following terms does the author supply a definition?
(A) "mantle’’ (line 3)
(B) "anchor’’ (line 10)
(C) "sound-wave guiding systems’’ (line 10)
(D) "bedrock’’(line l2)