[ServiceContract(Namespace = "http://Microsoft.ServiceModel.Samples", SessionMode=SessionMode.Required)] public interface ICalculator{ [OperationContract] double AddTo(double n); }

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)] public class CalculatorService : ICalculator { double result = 0.0D; public double AddTo(double n) { return result+=n; } }

a and b both

None of the above