A stone and a wooden block are released simultaneously from rest in a tall, evacuated tube. They reach the bottom together because
(a) The heavier stone pulls the lighter block downward through mutual gravitational attraction, so they keep pace with each other.
(b) Their accelerations are equal and independent of mass, so both bodies fall together in vacuum.
(c) Inside the tube an upward buoyant force just balances each object’s weight, making their net forces equal and allowing them to arrive together.
(d) Because air resistance is absent, the upward and downward forces on each object balance to zero, so both objects fall together at the same rate.