What does Hess's law state?

A. The enthalpy of a reaction can be determined only from single-
step reactions.

B. The enthalpy of a reaction is the sum of the enthalpies of
intermediate reactions.

C. The enthalpy of a reaction is positive if it is endothermic and
negative if exothermic.

D. The enthalpy of a reaction can be measured from heat produced in
a calorimeter.