A、<span style="font-family: 宋体; line-height: 24px;">必定都是急变流</span>
B、<span style="font-family: 宋体; line-height: 24px;">必须都是急变流</span>
C、<span style="font-family: 宋体; line-height: 24px;">不能出现急变流</span>
D、<span style="font-family: 宋体; line-height: 24px;">可以出现急变流</span>