天然气管天道置换空气以保证管线内天然气中含氧量小于（）.
A.1%
B.2%
C.3%
D.5%
参考答案：B
Li Ling doesn't want to _____ her friends _____. So he always helps them in time. [ ]
A. take; up
B. let; up
C. let; down
蒙古族群众喜爱（）。
A.红色
B.白色
C.蓝色
D.紫色
