A pebble fell freely from a certain height above the ground, and a photographer just caught its trajectory of falling ab. Amateurs use scales.

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 average speed of AB segment 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=2022× 10=20m。

So, C is correct.

So choose: C.