B, Xiao Fang is in the process of rising after taking off, and is only affected by gravity, with downward gravity acceleration, in a state of complete weightlessness, so B is wrong.

C, when the small square takes off, the ground should give people an upward supporting force, which is greater than human gravity, so people can have upward acceleration and upward movement, so C is correct.

D, in the process of Xiao Fang's descent, there is downward gravity acceleration only under the action of gravity, so D is wrong.

So choose AC.