affirmed.

　　[D] A theory is proposed, considered and then amended.

　　4. 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] Ships are another possibility, but they would need to stop every 50 kilometers to lower measuring instruments.

　　[B] To understand how ocean acoustic tomography works, it is necessary to know how sound travels in the ocean.

　　[C] Timekeeping in medical tomography must be precise because the changes in travel time caused by density fluctuations are slight.

　　[D] These variations amount to only about 2 to 3 percent of the average speed of sound in water, which is about 1500 meters per second.

　　5. The author mentions EL Nino primarily in order to emphasize .

　　[A] the difficulty of measuring the ocean’s large�瞫cale currents.

　　[B] the variability of mesoscale phenomena.

　　[C] the brief duration of weather patterns.

　　[D] the possible impact of mesoscale fields on weather conditions.

　　核心詞匯

　　circulation n. 流通,循環(huán),發(fā)行量

　　identify v.識別;(with)把…和…看成一樣,打成一片

　　dominate v. 支配,占優(yōu)勢

　　mesoscale n. & a.[氣] 中尺度(的) ,中等規(guī)模的

　　fluctuate v. 變動,上下,動搖 v. 使動搖

　　velocity n. 速度,速率

　　phenomenon n. 現(xiàn)象(pl.phenomena)

　　equatorial a. 近赤道的,赤道的

　　prohibitive a. 禁止的,抑制的,類同禁止的

　　conventional a. 普通的,常見的,習(xí)慣的,常規(guī)的

　　feasible