题目

Only when he started to explain        the reason for this. A. she realized                     B. did she realize C. she had realized                 D. had she realized 答案：B 甲、乙两同学同时从山脚开始爬山，到达山顶后立即下山，在山脚和山顶之间不断往返运动，已知山坡长为360 m，甲、乙上山的速度比是6∶4，并且甲、乙下山的速度都是各自上山速度的1.5倍，当甲第三次到达山顶时，则此时乙所在的位置是________．