The reason that explains why Americans living in the western United States supported the passage of the law in this question is the
"Chinese Exclusion Act, 1882 concern about competition for limited jobs."
This is based on the fact that during this period, the number of Chinese immigrants was increasing in the Western Part of the United States.
Many of these Chinese immigrants are working at a lower rate compared to a typical American and thereby reducing the number of available vacancies for the Americans.
Therefore, due to limited available jobs, many Americans living in the western United States supported the law's passage in this question.
Hence, in this case, the correct answer is option A. "Chinese Exclusion Act, 1882 concern about competition for limited jobs."
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