A, let the length of the ladder be L, and the inclination angle of AB be α. If f is the force exerted by the wall on the upper end of ladder B, then according to the moment balance:

FLsinα=GL2cosα+Phcotα, what? F=PcotαLsinαh+ 12Gcotα。 According to mathematical knowledge, F and H are linear and consistent with Figure B, so A is correct.

B. When people walk up the ladder from the bottom A at a uniform speed, the force exerted by people on the ladder is equal to the gravity of people and remains unchanged. So, B is wrong.

C, D For the overall study of the escalator, the horizontal direction is affected by the force F of the wall and the static friction F of the ground, and the vertical direction is affected by the total gravity P+G and the supporting force N of the ground. According to the equilibrium condition, N=P+G, f=F, so n remains unchanged. Figure b can't be an elastic image of a ladder on the ground. Figure b may be the friction force exerted by the ladder on the ground. So, C is wrong and D is right.

So choose AD.