________ is used to control the fission reaction rate in a nuclear reactor as it absorbs neutron without itself getting into fissioning. A. Heavy water B. Graphite C. Water D. Boron

Control rods of boron (or cadmium) absorb neutrons without undergoing fission, so lowering or raising them controls the chain reaction. Heavy water and graphite are moderators, which slow neutrons down rather than absorbing them.