洁净室应维持一定的（）
A.负压
B.正压
C.压差
D.压力
E.高压
参考答案：B
I didn't know when they again. [ ]
A. came
B. were coming
C. had come
D. had been coming
未满2周岁的婴儿随父母乘飞机旅行，按成人票价的（）付费。
A、10%
B、50%
C、80%
D、100%
