—Does Liu Hui serve in the army?

—No, but he ________ in the army for three years.

A．served

B．has served

C．is serving

D．would serve

UE随机接入时的过程中，UE通过（）建立上行同步，根据接受到的（），选择上行SYNC码，并以估计的时间和功率发送。