(第4空)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 ( )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.