Read this paragraph from the text:

The Pony Express was the first rapid transit and the first fast mail line across the continent from the Missouri River to the Pacific Coast. It was a system by means of which messages were carried swiftly on horseback across the plains and deserts, and over the mountains of the far West. It brought the Atlantic coast and the Pacific slope ten days nearer to each other.

Which words suggest a central idea for this section?

A. Rapid, fast, swiftly

B. Continent, coast, system

C. Plains, deserts, mountains

D. Mail, messages, nearer