Which of the following elements is commonly used as a primary material for constructing control rods in nuclear reactors?

Control rods are used in nuclear reactors to govern the rate of fission of uranium or plutonium by absorbing neutrons. Materials with high neutron-absorption cross-sections such as Cadmium ($\text{Cd}$), Boron, or Hafnium are chosen for this purpose. Graphite and Beryllium are typically used as moderators.