Locomotive pulling cars

A locomotive pulls a series of railroad cars. Which is true?

A. The train moves forward because the locomotive pulls forward slightly harder on the cars than the cars pull backward on the locomotive.
B. Because action equals reaction, the locomotive cannot pull the cars--the cars pull backward just as hard as the locomotive pulls forward.
C. The locomotive gets the cars to move by giving them a tug during which the force on the cars is momentarily greater than the force exerted by the cars on the locomotive.
D. The locomotive's force on the cars is as strong as the force of the cars on the locomotive, but the frictional force on the locomotive is forward and large while the backward frictional force on the cars is small.
E. The locomotive can pull the cars forward only if it weighs more than the cars.