TOEFL Reading: ETS-TOEFL阅读机经 - J1NF7_OTZQ1Y43Y7L$

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Because the lungs of these fish were able to provide only a small amount of oxygen, these fish obtained most of their oxygen through their gills during periods of drought. B. During periods of extended drought, these fish used their lungs to increase their intake of oxygen beyond the levels absorbed by the gills in normal times. C. Although these fish primarily used their gills to obtain oxygen, they used their lungs to obtain oxygen from the air when there was not enough in the water. D. During periods of extended drought, the gills became an auxiliary breathing device and the lungs became the main source of oxygen for these fish.