A concrete dam is made much thicker at the bottom than at the top. Which statement best explains this design choice?
(a) A greater wall thickness at the bottom allows the dam to push harder back against the water.
(b) A larger area at the bottom lowers the stress per unit area where the water pressure is greatest.
(c) The thickness is increased to raise the dam’s centre of gravity above the water level.
(d) Because water pressure becomes weaker with depth, extra material is needed at the bottom to balance buoyancy.