Make the interval math steps to explain the next text: The average photon number of the thermal field is calculated as /bar (n)=(:hat(n):)=Tr((hat(n))hat(rho )(Th))=∑(n=0)([infinity] ) (:n|(hat(n))h
a) Tr((hat(n))hat(rho )(Th))
b) ∑(n=0)([infinity] ) (:n|(hat(n))h
c) (:hat(n):)
d) None of the above