GRE Reading Comprehension: ETS-GRE阅读ETS - LSR5OUJ45V6W7SXK7

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