Which interface should you implement in the data contract class?

You are developing a data contract for a Windows Communication Foundation (WCF)
service. The data in the data contract must participate in round trips. Strict schema validity
is not required. You need to ensure that the contract is forward-compatible and allows new
data members to be added to it. Which interface should you implement in the data contract
class?

A.
IExtension<T>

B.
IExtensibleObject<T>