14. The force that causes the bed in the fluidization experiment to expand is (circle the correctmost correct answer): a) The drag force of the air b) The buoyance force C) The gravity force d) All of the above 15. The bed height after reaching the minimum fluidization velocity was (circle the correctmost correct answer): -a) Constant b) Constant then increasing c) Constant then decreasing d) Linearly increased c) Linearly decreased