The correct answer is: C. within the middle third of base.
A gravity dam is a type of dam that uses its own weight to resist the force of water. The maximum compressive force in a gravity dam is produced within the middle third of the base because this is where the water pressure is greatest. The water pressure is greatest at the middle third of the base because the water is pushing against the dam from all sides. The water pressure is less at the toe and heel of the dam because the water is only pushing against the dam from one side.
The toe of a dam is the point where the dam meets the ground. The heel of a dam is the point where the dam meets the reservoir. The middle third of the base is the area between the toe and heel of the dam.
The maximum compressive force in a gravity dam is important because it determines the strength of the dam. If the maximum compressive force is too great, the dam could collapse.