循环气过滤器的滤袋在更换时一般需要用CO2进行失活处理,使用CO2浓度一般为()。
A、10-20%
B、40-50%
C、60-70%
D、90-99%
参考答案:D
Big companies usually have a lot of branch offices _____ in different parts of the world. [ ]
A. to have operated
B. be operated
C. operating
D. having operated
花境