Martavis is installing a smart thermostat into a home built in the 1950s. The home still has the original steam boiler as its heat source and currently is set using the original dial thermostat. After connecting all of the old wires to the smart thermostat, Martavis notices that it doesn't turn on. What is the most likely issue with the installation?

A. The new smart thermostat is defective.
B. There's no C wire since the original thermostat is older.
C. There's no R wire since the home doesn't have a central air system.
D. The new smart thermostat isn't compatible with this type of boiler.