A hollow plastic ball of fixed volume floats in freshwater. Sand is slowly poured into the cavity until the ball just begins to sink. Which of the following best explains why adding sand causes the ball to sink?
(a) The sand forces air out of the cavity, and it is the trapped air that provides most of the upward force; with less air, the upward force becomes too small.
(b) Adding sand causes the ball to compress slightly and therefore reduces its volume; with less volume, the buoyant force drops below the ball’s weight.
(c) The added sand increases the ball’s average density until it exceeds that of water, so the weight of water displaced is insufficient to balance the ball’s weight.
(d) The added sand increases the ball’s mass, but it also increases the downward pressure the water exerts on the top of the ball more than on the bottom, producing a net downward force.