电力系统在很小的干扰下能独立恢复它的初始状态的能力，称为什么？
参考答案：静态稳定。
No one knows exactly when our ancestors started talking，but new evidence suggests _______might have happened a long time ago．
A．which
B．what
C．it
D．they
防止炉管结焦的措施是什么？
