GRE Reading Comprehension
In medical tomography x-rays map the human body's density variations (and hence internal organs); the information from the x-rays, transmitted through the body along many different paths, is recombined to form threedimensional images of the body's interior. It is primarily this multiplicative increase in data obtained from the multipath transmission of signals that accounts for oceanographers' attraction to tomography.
Researchers reasoned that low-frequency sound waves, because they are so well described mathematically and because even small perturbations in emitted sound waves can be detected, could be transmitted through the ocean over many different paths and that the properties of the ocean's interior could be deduced on the basis of how the ocean altered the signals. Their initial trials were highly successful, and ocean acoustic tomography was born.
Which of the following, if presented as the first sentence of a succeeding paragraph, would most logically continue the discussion presented in the passage?
- A Timekeeping in medical tomography must be precise because the changes in travel time caused by density fluctuations are slight.
- B To understand how ocean acoustic tomography works, it is necessary to know how sound travels in the ocean.
- C Ships are another possibility. but they would need to stop every 50 kilometers to lower measuring instruments.
- D These variations amount to only about 2 to 3 percent of the average speed of sound in water, which is about 1,500 meters per second.
- E The device used in medical tomography emits a specially coded signal, easily distinguishable from background noise
Correct Answer: B