A.[ServiceContract]public Class Service1{[Operation Contract]double Add（double n1，double n2）}
B.[ServiceContract]public Interface IService1{[Operation Contract]double Add（double n1，double n2）}
C.[ServiceContract]public Interface IService1{double Add（double n1，double n2）}
D.public Interface IService1{[Operation Contract]double Add（double n1，double n2）}