Helicopters can operate at a height between 1 and 10 meters with 10 meters distance from obstacles, but its flying speed should not exceed _______ km/h.
A.10
B.15
C.20
D.30
正确答案是B
Helicopters can operate at a height between 1 and 10 meters with 10 meters distance from obstacles, but its flying speed should not exceed _______ km/h.
A.10
B.15
C.20
D.30
正确答案是B