A pebble fell freely from the roof, and a photographer just caught its trajectory of falling AB. The amateur measured it with a ruler.

As can be seen from the figure, the length of AB is 2cm, that is, 0.02m, and the exposure time is 1 1000s.

So the size of the average speed of AB section is:

. v=xt=0.020.00 1=20m/s，

Because time is extremely short, the instantaneous speed of the moment corresponding to point A is about 20m/s,

According to the relationship between the velocity and displacement of free falling body v2=2gh:

H = V22g = 40020m = 20m。 So C is right and A, B and D are wrong.

So choose: C.