燃气管道绝热层要具有一定的强度，一般应能承受（）MPa的压力。
A、0.1
B、0.3
C、0.5
D、0.7
参考答案：B
Mr. Wang decided to devote all his time_______ his spoken English before going abroad.
A．improve
B．to improve
C．improving
D．to improving
距离保护装置对振荡闭锁有什么要求？
