管类乐器是靠_________的振动而发声的；弦类乐器是靠__________的振动而发声的；而口琴是靠_______的振动而发声的。
空气柱；弦；簧片
The newly-married couple quarreled so much that they reached the point _______ they had to separate from each other.
A．when
B．where
C．which
D．that
工程资料管理的保证有组织保证、制度保证、设备保证和( )。
A．经济保证
B．环境条件保证
C．安全保证
D．环节保证
