The in-flight personnel on board an unpressurized aircraft should start to use oxygen at the altitude of _______ m.
A.3600
B.4000
C.4500
D.5000
正确答案是B
The in-flight personnel on board an unpressurized aircraft should start to use oxygen at the altitude of _______ m.
A.3600
B.4000
C.4500
D.5000
正确答案是B