SW-220K型转向架轴距为（）。
A.2100mm
B.2200mm
C.2400mm
D.2500mm
参考答案：D
IEEE802参考模型将（）层分为逻辑链路控制子层与介质访问控制子层。
He asked a lot of questions, and none of ______ was easy to answer.
A．them
B．which
C．what
D．that
