这三句分别选什么,为什么?有什么语法错误
1.Ihavethreebrothers,______aredoctors.
2.Ihavethreebrothers,and______aredoctors.
A:twoofthemB:twoofwhomC:TwoofthemD:Twoofthem
3.Heboughtamapandwith___heaouldfindthepostoffice.
A.itBwhichC.oneDwhat