下列各项中，属于变动成本的有()。

A.按平均年限法计算的厂房折旧费

B.直接材料成本

C.生产工人计件工资

D.企业财产保险费

I don't understand what the engineer means, but I've got _____ rough idea of _____ project plan.[ ]

A. the; a

B. 不填; the

C. the; 不填

D. a; the