Which of the following, if presented as the first sentence of a succeeding paragraph, woul
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 i 500 meters per second.