Stroboscopic photography is a commonly used experimental method to study the motion of objects. Photography is carried out in a dark room, and the flash flashes every once in a while, and the negative

In Figure B (1), the interval between adjacent balls is the same, and the interval distance is also the same, so the ball B moves in a straight line at a uniform speed; In Figure A, the interval between adjacent balls is the same, and the interval distance is increasing, indicating that ball A is accelerating;

(2) The mass and gravity of the two balls are equal, both of which are G, and the ball A is in an accelerated motion state, so its resistance is less than its gravity, that is, f < G；; Ball b is in a state of uniform linear motion and is subjected to a balancing force, and the resistance is equal to gravity, that is, F'= G, so f' > F.

So, the answer is: b; B.