By the 1850s, San Francisco

A) had lost its pre-eminence to new metropolitan centers in California.
B) was merely a small rural settlement.
C) enjoyed prosperity almost exclusively due to the cotton trade.
D) had become the West Coast gateway to the interior.

By the 1850s, San Francisco had become the West Coast gateway to the interior. It was a major hub for transportation, trade, and commerce, connecting the Western United States to the rest of the country and the world. The city's population grew rapidly and it became one of the largest and most important cities in the United States.