Two Use Methods of Moving Pulley

As can be seen from the figure, the pulley used in Figure A has a fixed shaft, and the pulley is a crown block;

The pulley used in Figure B, whose shaft moves with the object, is a moving pulley;

Because the power arm and resistance arm of the crown block are equal to the radius of the pulley, the crown block is essentially an equal arm lever, so the advantage of using armor is that it can change the direction of force.

So, the answer is: a; b； You can change the direction of force.