If it were not for the fact that you _____ ill, I would ask you to do this right now.

A．were

B．had been

C．are

D．should be

标准偏差是统计上衡量（）的指标，通常用在数据为正态分布时。标准偏差越大，数据的散布程度也就越大。恒定的有68.26%的数据在正负一个标准方差内，99.73%的数据在正负3个标准方差内。