You are required to migrate your 11.2.0.3 database as a pluggable database (PDB) to a multitenant container database (CDB). The following are the possible steps to accomplish this task: 1. Place all the user-defined tablespace in read-only mode on the source database. 2. Upgrade the source database to a 12c version. 3. Create a new PDB in the target container database. 4. Perform a full transportable export on the source database with the VERSION parameter set to 12 using the expdp utility. 5. Copy the associated data files and export the dump file to the desired location in the target database. 6. Invoke the Data Pump import utility on the new PDB database as a user with the DATAPUMP_IMP_FULL_DATABASE role and specify the full transportable import options. 7. Synchronize the PDB on the target container database by using the DBMS_PDS.SYNC_ODB function. Identify the correct order of the required steps.（）

A. 2，1，3，4，5，6
B. 1，3，4，5，6，7
C. 1，4，3，5，6，7
D. 2，1，3，4，5，6，7
E. 1，5，6，4，3，2