单项选择题 AnetworkadministratorprovidesanIPaddressof128.135.0.142andasubnetmaskof255.0.0.0.Onceapingisattempted，itfails.Whichofthefollowingoptionsisthemostprobablecauseofthefailedping？（）

单项选择题 WhichofthefollowingoptionsisrepresentativeofaClassBaddress？（）

单项选择题 The local host on a small network is named Earth. There is also a remote host on the network named Saturn.  The system administrator of the network is in the /u/tmp directory of Earth， and starts an ftp to Saturn as root.  Which of the following sequence of commands issued from the local host， Earth， will：（）。  1.Check to see if there is an a.out file on remote host Saturn.   2.Change to the /u/new directory on Earth.  3.Transfer a.out from Earth to Saturn.