单选题 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

【正确答案】 D

【答案解析】与菌根真菌共生的植物对其他有害真菌有抵抗作用，其原因，文中提到三种意见，究竟哪一个是正确的，至今仍无定论，所以选择D“没有一个令人满意的解释”。