Oracle还原并恢复undo表空间的一个数据文件，对下面的步骤正确排序：5,4,3,2 (1,6,7 不需要) 1.Bring the file online. 2.Open the database. 3.Recover the damaged file. 4.Restore the damaged file. 5.Start the database in mount mode. 6.Start the database in nomount mode. 7.Take the damaged file offline.