根瘤菌
参考答案：与豆科植物共生，形成根瘤并固定空气中的氮气供植物营养的一类杆状细菌。
They didn't leave the station ___ they got on the train. [ ]
A. until
B. by
C. after
D. at
简述共同侵权的概念和特征。
