Homework ________, I turned on the computer and tried to do some online shopping.

A．finishing

B．finished

C．to finish

D．finishes

以林区为抽样总体，按照预定因子分组，在组内随机或系统抽取单元组成样本，用以估计总体的方法叫（）。

A.简单随机抽样

B.系统抽样

C.分层抽样

D.二阶抽样