Race against time Dora, run!

A railway crossing, safety first, obviously can not race against time, does not meet the requirements;

The slogan B is not suitable for production enterprises and can be used in other special places.

C there is competition in the examination room, and the pursuit of success should be based on encouragement, which does not meet the requirements;

D language is concise and clear-cut, which is applicable here.

Answer: D.