膨胀罐内的氮气覆盖系统可以防止空气进入系统造成热媒（）。
A.氧化
B.还原
C.燃烧
D.失效
参考答案：A
He________English since he graduated from college and he is now even more interested in teaching it. [ ]
A. taught
B. was teaching
C. has been teaching
D. had taught
棉花的一生有哪几个生育时期？
