-It's the second time that I ________ to Shanghai.

-What great changes！It's ten years since I ________ it last time.[ ]

A．have been; left  

B．had been; left

C．am; had left  

D．come; had left

在高速场景下，终端移动速度很快，信道质量变化快，如果TTT（TimetoTrig）设置较大，测量小区的信道质量与TTT时刻后的信道质量很难匹配，容易导致切换失败，因此，TTT的设置会影响切换成功率。（）