For a given characteristic in a construction process, the standard deviation associated with production variation is 0.8", and the standard deviation associated with measurement variation is 0.6". The total standard deviation for this characteristic is: A 1.4" B 1.2". C 1.0" D 0.8" E 0.2" 如何计算这个标准差呢,它的公式到底是什么啊,我不会