工作内容复杂、投资额大的咨询项目、咨询服务的采购方式是( )。

A．有限竞争性招标
B．竞争性招标
C．直接谈判采购
D．邀请招标

To finish the homework, he spent four hours _______ in the study.

A．locking

B．locked

C．locked himself

D．to lock