On the construction site, the crane can send the needed bricks to the roof in a few minutes, and it will take several hours if people directly use the pulley block to lift them. as a result of

A. Cranes and people do the same work on bricks. The crane takes less time, works fast and has high power. Cranes and people use pulleys to transport the same bricks to the roof and do the same work. This is not in line with the problem.

B, the crane and people do the same work on bricks, and the crane takes less time, does work quickly and has high power.

C, cranes and people do the same useful work, cranes do more extra work, and the proportion of useful work in the total work is small, and the mechanical efficiency is low.

D, cranes and people do the same useful work, cranes do more extra work, and the proportion of useful work in the total work is small, and the mechanical efficiency is low. Cranes and people use pulleys to transport the same bricks to the roof and do the same work, which does not meet the question.

So choose B.