在工程项目管理组织结构中，从事和完成具体项目任务的是( )。
A．协调层
B．执行层
C．操作层
D．决策层
参考答案：C
Mary used to ______ computer games outside on weekends, but now he is used to ______ his
homework at home. [ ]
A. playing, do
B. playing, doing
C. play, doing
D. play, does
城市水体污染的控制和治理技术有哪些方法？
