2、下列说法不正确的是 。 【 】
A）力多边形不自行封闭，则对应的平面汇交力系一定不平衡。
B）力偶在任何坐标轴上投影的代数和恒为零。
C）合力在某轴上的投影等于各分力在同一轴上投影的代数和。
D）力可以平移到刚体内的任意一点。
Which of the following components is NOT used in 10Base5 Ethernet ?
A. (a) Transceivers
B. (b) Terminators
C. (c) Optical Fiber
D. (d) Repeaters
E. (e) Coaxial cable
Which of the following are correct for CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol ?
A. (a) Listen (carrier sense) before transmit.
B. (b) If channel is sensed idle, then transmit entire frame even a collision is detected.
C. (c) If channel is sensed idle, then transmit a frame and stop the transmission when a collision is detected.
D. (d) If channel is sensed busy, then defer transmission for a random time.
E. (e) If channel is sensed busy, then defer transmission until the channel is idle.
Which of the following are correct when a “collision” happens for CSMA/CD protocol ?
A. (a) The entire frame will still be completely transmitted.
B. (b) A jamming sequence will be sent by the sending host to enforce the collision.
C. (c) The transmission is stopped and after a random delay, the frame is transmitted again from the “carrier sense” step.
D. (d) The transmission is stopped and after a random delay, the frame is transmitted again immediately.
E. (e) The transmission is stopped. But the frame is transmitted again as long as the channel becomes idle.
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