A, according to the composition of the movement, the direction of the closing speed and the direction of the closing acceleration are not in the same straight line, and the direction of the acceleration remains the same, so the fireman does a curve movement with uniform speed change. So A and C are wrong and B is right.

D, the movement of firefighters is decomposed into horizontal direction and vertical direction, and the final speed in the horizontal direction is known as the sum of the uniform retreating speed and the speed in the horizontal direction along the ladder. Because the horizontal speed along the ladder direction changes, so does the speed of firefighters in the horizontal direction. So, d is wrong.

So choose B.