以下产品属于中低端二层以太网交换机的有（）.

A.S5000系列千兆以太网交换机

B.S3000系列百兆以太网交换机

C.S3500快速智能交换机

D.S2000系列交换机

In 1969 ____took his biggest step away from the earth. Americans first walked on the moon.This is certainly just a ____thought.

A．he,moving

B．people ,surprising

C．an astronaut, interesting

D．man ,beginning