Definition: In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.
Input: Question: Which of the nearby counties of the "Tennessee Valley" has the largest population? Passage:According to the United States Census 2013 estimate, the combined statistical area has a population of 1,313,105. It is the 42nd largest population sub-region in the United States, and the largest population region in Alabama, constituting roughly 1/4 of the state's population. It is the largest population region in Central Alabama. The northern counties of the metropolitan area overlap North Alabama known locally as the "Tennessee Valley" making it the largest metropolitan area in that region as well. Nearby counties Tuscaloosa, Etowah, and Calhoun, while not officially a part of Greater Birmingham, contribute significantly to the region's economy. The Birmingham media market covers these counties as well. Birmingham is part of the Piedmont Atlantic Megaregion containing an estimated 19 million people, while many residents also consider themselves part of the Deep South. It is classified as Southeast by the U.S. Census and also falls in the geographic area of the Upland South due to its location at the southern terminus of the Appalachian foothills. The entire MSA and CSA are within the congressional Appalachian Regional Commission's definition of Appalachia.
Links: a. Tuscaloosa County, Alabama b. Appalachia c. Southeastern United States d. Piedmont Atlantic Megaregion
Output:
a