Question: What region of the country is the US state having the county containing Oak Grove located?

full_context: Document 1: Geographic center of the United States: Its position as located in a 1918 survey is located at 39 ° 50 ′ N 98 ° 35 ′ W ﻿ / ﻿ 39.833 ° N 98.583 ° W ﻿ / 39.833; - 98.583 ﻿ (Geographic Center of the Contiguous United States), in Kansas about 2.6 miles (4.2 km) northwest of the center of Lebanon, approximately 12 miles (19 km) south of the Kansas - Nebraska border.
Document 2: Oakbrook, Kentucky: Oakbrook is a census-designated place (CDP) in Boone County, Kentucky, United States. The population was 9,036 at the 2010 census.
Document 3: Corn Belt: The Corn Belt is a region of the Midwestern United States that, since the 1850s, has dominated corn production in the United States. More generally, the concept of the ``Corn Belt ''connotes the area of the Midwest dominated by farming. Many towns in this area are connected to powerful farm organizations with lobbying power.
Document 4: Crystal City, Missouri: Crystal City is a city in Jefferson County, Missouri, United States. The population was 4,855 at the 2010 census. It was 4,247 at the 2000 census.
Document 5: North Dakota: North Dakota is in the U.S. region known as the Great Plains. The state shares the Red River of the North with Minnesota to the east. South Dakota is to the south, Montana is to the west, and the Canadian provinces of Saskatchewan and Manitoba are to the north. North Dakota is situated near the middle of North America with a stone marker in Rugby, North Dakota marking the ``Geographic Center of the North American Continent ''. With an area of 70,762 square miles (183,273 km), North Dakota is the 19th largest state.
Document 6: Fremont Hills, Missouri: Fremont Hills is a city in Christian County, Missouri, United States. The population was 826 at the 2010 census. It is part of the Springfield, Missouri Metropolitan Statistical Area.
Document 7: Geographic center of the United States: Its position as located in a 1918 survey is located at 39 ° 50 ′ N 98 ° 35 ′ W ﻿ / ﻿ 39.833 ° N 98.583 ° W ﻿ / 39.833; - 98.583 ﻿ (Geographic Center of the Contiguous United States), about 2.6 miles (4.2 km) northwest of the center of Lebanon, Kansas, approximately 12 miles (19 km) south of the Kansas - Nebraska border.
Document 8: Southwest City, Missouri: Southwest City is a city in McDonald County, Missouri, United States. The population was 937 at the 2010 census, at which time it was a town. It is part of the Fayetteville–Springdale–Rogers, AR-MO Metropolitan Statistical Area and is located in the southwestern corner of the state of Missouri.
Document 9: Midwestern United States: The Midwestern United States, also referred to as the American Midwest, Middle West, or simply the Midwest, is one of four geographic regions defined by the United States Census Bureau. It occupies the northern central part of the United States of America. It was officially named the North Central region by the Census Bureau until 1984. It is located between the Northeastern U.S. and the Western U.S., with Canada to its north and the Southern U.S. to its south.
Document 10: Mount Olivet, Kentucky: Mount Olivet is a home rule-class city in and the county seat of Robertson County, Kentucky, United States, located at the junction of U.S. Route 62 and Kentucky Route 165. The population was 299 at the 2010 United States census.
Document 11: Oak Grove, Kentucky: Oak Grove is a home rule-class city adjacent to the Fort Campbell army base in Christian County, Kentucky, in the United States. The population was 7,489 as of the 2010 U.S. Census. It is part of the Clarksville, Tennessee metropolitan area.
Document 12: St. Charles, Missouri: St. Charles is a city in, and the county seat of, St. Charles County, Missouri, United States. The population was 65,794 at the 2010 census, making St. Charles the ninth - largest city in Missouri. It lies to the northwest, and is a suburb of St. Louis, Missouri on the Missouri River.
Document 13: Christian County, Missouri: Christian County is part of the Springfield, MO Metropolitan Statistical Area. Between 2000 and 2010, it was the fastest growing county in Missouri and one of the fastest growing in the nation as the county became more suburban due to the booming growth in Springfield.
Document 14: Coal Grove, Ohio: Coal Grove is a village in Lawrence County, Ohio, United States, along the Ohio River. The population was 2,165 at the 2010 census. It borders the city of Ironton and lies across the Ohio River from Ashland, Kentucky. It is linked to Ashland by the Ben Williamson Memorial Bridge and Simeon Willis Memorial Bridge.
Document 15: Oak Grove, Sumner County, Tennessee: Oak Grove is a census-designated place and unincorporated community in Sumner County, Tennessee, United States. Its population was 231 as of the 2010 census.
ground_truth: Midwestern United States
sub_queries: Oak Grove >> located in the administrative territorial entity Answer: Christian County
Christian County >> located in the administrative territorial entity Answer: Missouri
what region of the country is Missouri in Answer: Midwestern United States