A project management team is evaluating the causes that might contribute to unsatisfactory performance and quality. Which of the following statements is not true ？（）
A．Normal process variation is attributable to random causes and sometimes also called "white noise"
B．Special causes are easier to predict and handle than random causes(also called common causes)
C．Special causes are unusual events which are difficult to foresee and mostly produce outliers
D．A process can be optimized to limit the bandwidth of variations due to random causes