In multipoint networks , there are tree persistence methods when a station finds a channel busy.in the 1-persistent method,after the station finds the line idle,it sends its frame immediately.this method has the （请作答此空） chance of collision because two or more stations may find the line （  ） and send their frames immediately. In the nonpersistent method , a station that has a frame to send （  ） the line . If the line is idle , it sends immediately If the line is not idle , it waits a （  ） amount of time and then senses the line again.The nonpersistent approach （  ） the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously The p-persistent approach combines the advantages of the other two strategies . It reduces the chance of collision and improves efficiency.

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