GRE Reading Comprehension: ETS-GRE阅读ETS - K1UP8L2EOKED7M5Z7

The main idea of the passage is that A. migration over land requires a simpler explanation than migration over water does B. the means by which animals migrate over water are complex and only partly understood C. the ability of migrant animals to keep track of time is related to their magnetic sense D. knowledge of geographic location is essential to migrants with little or no compass sense E. explanations of how animals migrate tend to replace, rather than build on, one another