Which two statements are true about adding a custom extension column to an existing table?

Which two statements are true about adding a custom extension column to an existing table? (Choose two.)

A.
You cannot add extension columns to interface tables.

B.
The new column name is automatically suffixed with _X.

C.
The new column name is automatically prefixed with X_.

D.
You must check out the appropriate project/object definition before adding a column.