Which code segment should you use to set the service behavior?

You are developing a Windows Communication Foundation (WCF) service.
One of the service operations contains the following code.

private static int counter = 0;
[OperationContract]
public void IncrementCount()
{
counter++;
}

You need to set a service behavior that prevents two or more threads from incrementing the counter variable at the same time.
Which code segment should you use to set the service behavior?

A.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Single)]

B.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession, ConcurrencyMode = ConcurrencyMode.Single)]

C.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple)]

D.
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall, ConcurrencyMode = ConcurrencyMode.Reentrant)]

Explanation:
“prevents two or more threads…at the same time” –> ConcurrencyMode.Single

ConcurrencyMode Enumeration
(http://msdn.microsoft.com/en-us/library/system.servicemodel.concurrencymode.aspx)

InstanceContextMode Enumeration
(http://msdn.microsoft.com/en-us/library/system.servicemodel.instancecontextmode.aspx)