The right answer is C. too many workers and not enough jobs in China. Chinese laborers were drawn to California’s goldfields, escaping from the Opium wars and looking for better opportunities, where nativists objected to their presence because of their poverty and their religion. Let's take a look at the other propositions:
A. high wages for immigrants in American factories - True - The promise of cheap land and good wages drew millions of immigrants to America.
B. a failed revolution in Germany - True - These immigrants included a large number of learned, cultured professional people—doctors, lawyers, teachers, engineers—some of them refugees from the failed German revolution of 1848.
D. crop failure and starvation in Ireland - True - After an epidemic of potato rot in 1845, called the Irish potato famine killed more than 1 million peasants, the flow of Irish immigrants to Canada and the United States became a flood.