You are modifying an existing Windows Communication Foundation (WCF) service that is defined as follows. SubmitOrder makes a call to another service. The ProcessMessage method does not perform as…

You are modifying an existing Windows Communication Foundation (WCF) service that is defined as follows. SubmitOrder makes a call to another service. The ProcessMessage method does not perform as expected under a heavy load. You need to enable processing of multiple messages. New messages must only be processed when the ProcessMessage method is not processing requests, or when it is waiting for calls to SubmitOrder to return. Which attribute should you apply to the MessageProcessor class?