A/O系统中的A段DO通常为（）；O段DO通常为（）
参考答案：0.2～0.5 mg/L；2～4 mg/L
Ping Pong is popular among China, _____ both young and old can _____. [ ]
A. about it; take part in
B. in it; take part
C. in which; take part in
D. in which; take part
银行业金融机构案件问责应当遵循什么原则？
