Answer:
A. Gold
Explanation:
After the Civil War, a rather big number of Chinese people emigrated to the United States in the middle of the California gold rush that took place during 1849 and 1882. On one side, most of these people emigrated to become U.S. citizens, on the other, some worked temporarily and came back home. As it was mentioned, they were initially captivated to travel to the U.S. by the gold rush in California.
