要说明白点为什么选那个答案a选项是正确答案b选项是我的答案1.wewillhavemodernequipment_____thingsthatarecomplicated.a.toassistuswithb.assistinguswith2.___fromtherestoftheworldforyears,hecould'tgetus

　　要说明白点为什么选那个答案a选项是正确答案b选项是我的答案

　　1.wewillhavemodernequipment_____thingsthatarecomplicated.

　　a.toassistuswithb.assistinguswith

　　2.___fromtherestoftheworldforyears,hecould'tgetusedtocitylifeagainwhenhereturned.

　　a.Separatedb.Havingseparated

　　3.Beforeyouleave,makesuretokeeptheboat_____tothetreeoverthere.

　　a.tiedb.beingtied

　　4.Only_____toSouthAfricabefore.

　　a.Tomhasbeenb.hasTomgone

　　不看我要求的都别回了我都说了a选项就是正确答案我要知道的是为什么选a而不选b看不懂要求的别回