The signal must be given in such a precise way that the pilot cannot confuse. 信号必须以如此精确的方式发出，以免飞行员混淆。
A.You must give the obvious signal in a certain way in order to be seen by the pilot. B.You must stand on a specific road, that is a clear signal for the pilot C.You must give the pilot the clear sign al that is not misunderstood by the pilot '正确答案C
