生物固氮主要由()与()两种微生物实现。
参考答案:非共生微生物、共生微生物
31.The play_____ next month aims mainly to reflect the local culture.
A.produced
B.being produced
C.to be produced
D.having been produced
肺泡上皮细胞有Ⅰ型细胞、()和()。