Seattle (i/siˈætəl/) is a West Coast seaport city and the seat of King County. With an estimated 662,400 residents as of 2015[update], Seattle is the largest city in both the state of Washington and the Pacific Northwest region of North America. In July 2013 it was the fastest-growing major city in the United States, and remained in the top five in May 2015 with an annual growth rate of 2.1%. The Seattle metropolitan area of around 3.6 million inhabitants is the 15th largest metropolitan area in the United States. The city is situated on an isthmus between Puget Sound (an inlet of the Pacific Ocean) and Lake Washington, about 100 miles (160 km) south of the Canada–United States border. A major gateway for trade with Asia, Seattle is the third largest port in North America in terms of container handling as of 2015.
Try to answer this question if possible (otherwise reply "unanswerable"): Of which Washington county is Seattle the center?
King County