Why Do Hot Water Heaters Lose Their Efficiency Quicker in Households

Why do hot water heaters lose their efficiency quicker in households with hard water?

A) The hard water calcium and magnesium ions corrode the heating element.
B) The hard water calcium carbonate deposits oxidize the inner tank wall.
C) The calcium carbonate on the inner surface of the hot water tank builds up to the point that it insulates the water from the heating element.
D) The hard water calcium ions exchange with iron ions from the inner metal wall which react with the water heater base to form rust and deteriorate the bottom of the tank.