—How is your sister now?

—______

A．She is thirty

B．She is a nurse

C．she’s much better

D．She’s Yang Jie

（）即把所有调查对象都按某种标志排列起来，按一定顺序、时间抽取样本。

A.等距抽样

B.多级抽样

C.聚类抽样

D.随机抽样