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答案：B
解析：
Choice B is the best answer. The author states that soil commonly is nitrogen-deficient and that nitrogen is often supplied by fertilizers, even though the fixed nitrogen released when legume plants with nitrogen-fixing nodules die can be used by other plants. This suggests that the fixed nitrogen those legume plants release is not enough to supply all of the nitrogen that is needed in soil; fertilizers are needed to address the remaining nitrogen deficiency.
