下列不属于生态系统的是 [ ]
A．一片农田
B．一块草地
C．一片森林
D．一群羊
D
The old engineer's eyes still shone bright in the wrinkled brown face and his step as he came across
the room was__________ ，though slow. [ ]
A. shaky
B．heavy
C casual
D．steady
1850 TSS-5C采用的是紧凑型机框设计，其机框的高度为（）
A、1U
B、2U
C、3U
