A, dry weather, pay attention to fire prevention, air humidity is small, fuel is easier to burn. This option does not meet the meaning of the question;

B, please fasten the driver's seat belt to prevent the injury caused by inertia. This option does not meet the meaning of the question;

C, rainy road slippery, drivers drive carefully. In rainy days, the friction between the wheel and the ground becomes smaller and the braking distance becomes a, which is in line with the meaning of the question;

6. Passengers waiting on the train station platform should stand outside the safety line, and the relationship between fluid velocity and pressure. This option does not meet the problem.

So choose C.