单项选择题

The author refers to a "conveyor belt"(L. 6, Para. 1) in order to ______.

A．illustrate the effects of movements in the upper layer of the earth
B．show how temperature differences depend on the positions of the continents
C．describe the complicated motions made possible by the ocean currents
D．account for the rising currents under min-ocean ridges
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单项选择题
According to the traditional view of the origin of the ocean basins, which of the following is sufficient to move the continental plates
A．Spreading of ocean trenches.
B．Movements of min-ocean ridges.
C．Sinking of ocean basins.
D．Differences in temperature under oceans and continents.
单项选择题
Which of the following, if it could be demonstrated, would most support the traditional view of ocean basin formation
A．The upper mantle of the earth behaves as a dense solid.
B．Movements usually occur along lines.
C．Sinking plates cool the surface of the earth.
D．The rising motion currents keep exact pace with them.
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