―What’s the matter? You look worried and restless.

―Oh, nothing really serious. As a matter of fact, I ______ of my graduation thesis. I have to hand it in soon.

A．am just thinking

B．just thought

C．have just thought

D．was just thinking

为调查我国国有企业在职职工的延迟退休意愿，分别从东部、中部和西部地区采用简单随机抽样方法各自抽取10个地级行政区域，然后在被抽中企业的所有在职职工发放问卷调查。该项调查中，除了简单随机抽样，还使用到的抽样方法有（）。

A.配额抽样 

B.分层抽样 

C.整群抽样 

D.系统抽样 

E.多阶段抽样