The VOR signals travel on a line-of-sight basis. This means _______ .
A.reception range for VOR signals is limited by line of sight
B.obstructions such as a building or the curvature of the earth can block the transmission and thus reduce the reception distance of the VOR signals
C.the VOR signals can occasionally be made unusable for navigation purposes by terrain features¬
D.all the above
正确答案是D