Question:  Marion is approximately 50 mi north of a city that is the third-most populous what in the U.S.?

Context articles: Knox County, Tennessee
Blue Flint Ethanol
Demographics of Mexico
Marion, Ohio
Santa Fe de Antioquia
Cambridge
Albany, New York
Atlee, Virginia
Columbus, Ohio
Rohnert Park, California

Knox County is a county in the U.S. state of Tennessee.
 As of the 2010 census, the population was 432,226, making it the third-most populous county in Tennessee.
 Its county seat is Knoxville, the third-most populous city in Tennessee.
Blue Flint Ethanol is an bioethanol producing company and a production plant with a same name, located in Underwood, North Dakota approximately 50 mi north of Bismarck.
 The plant is unique in the fact that rather than burning fuels, such as coal or natural gas to drive the production process, waste heat from electrical generation at the Coal Creek Station is used.
 The US$100 million plant is capable of processing 18 million bushels (460,000 metric tons) of corn to produce 50 million gallons (190 million liters) of ethanol each year, and is estimated to have a net annual economic impact of $160 million on the North Dakota economy, as well as the creation of approximately 40 new jobs to run the plant.
 In addition to producing ethanol the plant will also produce dry distillers grains, a byproduct of the distillation process which is used as animal feed.
 Of the 18 million bushels of corn used each year for feedstock, the majority will be grown in southeast North Dakota and brought in via rail, with the remaining one third being produced locally.
 In 2006 the Blue Flint Ethanol project was awarded the Project of the Year Award by Governor John Hoeven.
With a population of over 123 million in 2017, Mexico ranks as the 10th most populated country in the world.
 It is the most populous Spanish-speaking country and the third-most populous in the Americas after United States and Brazil.
 Throughout most of the twentieth century Mexico's population was characterized by rapid growth.
 Although this tendency has been reversed and average annual population growth over the last five years was less than 1%, the demographic transition is still in progress, and Mexico still has a large cohort of youths.
 The most populous city in the country is the capital, Mexico City, with a population of 8.9 million (2016), and its metropolitan area is also the most populated with 20.1 million (2010).
 Approximately 50% of the population lives in one of the 55 large metropolitan areas in the country.
 In total, about 78.84% of the population of the country lives in urban areas, meaning that only 21.16% live in rural areas.
Marion is a city in and the county seat of Marion County, Ohio, United States.
 The municipality is located in north-central Ohio, approximately 50 mi north of Columbus.
Santa Fe de Antioquia is a municipality in the Antioquia Department, Colombia.
 The city is located approximately 50 mi north of Medellín, the department capital.
 It has a population of approximately 23,000 inhabitants.
Cambridge ( ) is a university city and the county town of Cambridgeshire, England, on the River Cam approximately 50 mi north of London.
 At the United Kingdom Census 2011, its population was 123,867, including 24,488 students.
Albany ( ) is the capital of the U.S. state of New York and the seat of Albany County.
 Roughly 150 mi north of New York City, Albany developed on the west bank of the Hudson River, about 10 mi south of its confluence with the Mohawk River.
 The population of the City of Albany was 97,856 according to the 2010 census.
 Albany constitutes the economic and cultural core of the Capital District of New York State, which comprises the Albany-Schenectady-Troy, NY Metropolitan Statistical Area, including the nearby cities and suburbs of Troy, Schenectady, and Saratoga Springs.
 With a 2013 Census-estimated population of 1.1 million the Capital District is the third-most populous metropolitan region in the state and 38th in the United States.
Atlee or Atlee Station is an unincorporated community in northerneastern Hanover County in the Mid-Atlantic state of Virginia, United States.
 Atlee is located 11 mi north of Richmond on Route 637 approximately .6 mi north-northwest of the intersection of Route 637 and Virginia State Route 2.
 and approximately 50 mi south of Fredericksburg, Virginia
Columbus ( ) is the capital and largest city of the U.S. state of Ohio.
 It is the 14th-largest city in the United States, with a population of 860,090 as of 2016 estimates.
 This makes Columbus the third-most populous state capital in the United States, and the second-largest city in the Midwestern United States, after Chicago.
 It is the core city of the Columbus, Ohio, Metropolitan Statistical Area, which encompasses ten counties.
 With a population of 2,021,632, it is Ohio's third-largest metropolitan area.
Rohnert Park is a city in Sonoma County, California, United States, located approximately 50 mi north of San Francisco.
 The population at the 2010 United States Census was 40,971.
 It is an early planned city and is the sister city of Hashimoto in Japan.
 Sonoma State University, part of the California State University system, is located nearby.