问题：

[单选] A Windows Communication Foundation （WCF） solution exposes the following contract over an HTTP connection. Function GetData（） As String End Interface Existing clients are making blocking calls to GetData. Calls to GetData take five seconds to complete. You need to allow new clients to issue non-blocking calls to get the data， without breaking any existing clients. What should you do？（）

A .
B .
C . Generate a proxy class with asynchronous methods and use it for the new clients. 
D . Add a new endpoint to the service that uses a full-duplex binding and use it for the new clients.&e