What are the following further basics about defining key mappings?

What are the following further basics about defining key mappings?
A . Key mapping is required in communication if you work with unharmonized keys in your system landscape.
B . Key mapping can be used without any Customizing.
C . Review and enhance the basic mapping rules in customizing if required or you created your own Business Objects.
D . Support for ID-mapping, including key mapping of master data and value mapping of organizational data

Answer: A,B,C