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单项选择题

The passage suggests which of the following about the increased resistance to harmful root fungi that some Plants infected with mycorrhizal fungi seem to exhibit
A．There are at least three hypotheses that might account for the increase.
B．The plants that show increased resistance also exhibit improved nitrogen fixation.
C．Such increases may be independent of mycorrhizal infection.
D．It is unlikely that a satisfactory explanation can be found to account for the increas
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单项选择题 The influence of Greece and Rome became greater in the fourteenth and fifteenth century because ______.

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