17. The main idea of the passage is that
migration over land requires a simpler explanation than migration over water does
the means by which animals migrate over water are complex and only partly understood
the ability of migrant animals to keep track of time is related to their magnetic sense
knowledge of geographic location is essential to migrants with little or no compass sense
explanations of how animals migrate tend to replace, rather than build on, one another
18. It can be inferred from the passage that if the flock of birds described in lines 8-12 were navigating by compass sense alone, they would, after the storm, fly
east
north
northwest
south
southeast
19. In maintaining that migrating animals would need a fantastic map sense to determine their geographic position by celestial navigation, the author intends to express
admiration for the ability of the migrants
skepticism about celestial navigation as an explanation
certainly that the phenomenon of migration will remain mysterious
interest in a new method of accounting for over-water migration
surprise that animals apparently navigate in much the same way that human beings do
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