A:六角螺栓
B:紧定螺钉
C:自攻螺钉
D:膨胀螺栓
人类的历史变迁、世界的激烈竞争、祖国的百年沧桑,已经无数次地证明:落后就要挨打。他时刻警示着我们( )
The doctor ________the boy as soon as he was sent to the hospital.
关于声和电磁波的说法正确的是 ( )