96. Auxiliary boilers are divided into several classifications, one of which is __________.

Water-tube forced circulation is a recognized classification of auxiliary boilers, combining tube arrangement and water circulation method. Auxiliary boilers are classified by tube arrangement (water-tube vs fire-tube) and circulation method (natural vs forced). Water-tube boilers have water/steam inside the tubes, while fire-tube boilers have combustion gases inside the tubes. Forced-circulation boilers use pumps to move water, unlike natural circulation which relies on convection. Other options describe boiler types or circulation methods not used in standard auxiliary boiler classifications.