—I'm sorry. I'm late for the music presentation.

—Why are you so late? The presentation _______ for 10 minutes. [ ]

A. has begun

B. has stopped

C. has been over

D. has finished

一定条件下，下列物质在密闭容器内反应一段时间，测得反应产前后各物质的质量如下：

根据上表信息判断，下列说法中不正确的是[ ]

A．X、Z是反应物，Y、W是生成物  

B．a=15  

C．反应中Y、W两种物质变化的质量比为22：9  

D．Z为化合物