Task: Question Answering
Input (English): Passage: According to academic Rob Wilson, San Francisco is a global city, a status that pre-dated the city's popularity during the California Gold Rush. Such cities are characterized by their ethnic clustering, network of international connectivity, and convergence of technological innovation. Global cities, such as San Francisco, are considered to be complex and require a high level of talent as well as large masses of low wage workers. A divide is created within the city of ethnic, typically lower-class neighborhoods, and expensive ones with newly developed buildings. This in turn creates a population of highly educated, white-collar individuals as well as blue-collar workers, many of whom are immigrants, and who both are drawn to the increasing number of opportunities available. Competition for these opportunities pushes growth and adaptation in world centers.San Francisco has a diversified service economy, with employment spread across a wide range of professional services, including financial services, tourism, and (increasingly) high technology. In 2016, approximately 27% of workers were employed in professional business services; 14% in leisure and hospitality; 13% in government services; 12% in education and health care; 11% in trade, transportation, and utilities; and 8% in financial activities. In 2017, GDP in the five-county San Francisco metropolitan area grew 3.4% to $501 billion. Additionally, in 2017 the 14-county San Jose–San Francisco–Oakland combined statistical area had a GDP of $907 billion, ranking 3rd among CSAs, and ahead of all but 16 countries. As of 2017, San Francisco County was the 7th highest-income county in the United States (among 3,142), with a per capita personal income of $119,868. Marin County, directly to the north over the Golden Gate Bridge, and San Mateo County, directly to the south on the Peninsula, were the 5th and 9th highest-income counties respectively. 
Question: What county has the 9th highest-income county in the United States?
Output (English): San Mateo County.