Question: What vitamin is David P. Ballou known for studying?

Context paragraphs:
Vitamin B12: Vitamin B, also called cobalamin, is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system via the synthesis of myelin (myelinogenesis), and the formation of red blood cells.  It is one of eight B vitamins.  It is involved in the metabolism of every cell of the human body, especially affecting DNA synthesis, fatty acid and amino acid metabolism.  No fungi, plants, or animals (including humans) are capable of producing vitamin B. Only bacteria and archaea have the enzymes needed for its synthesis.  Some substantial sources of B include animal products (shellfish, meat), fortified food products, and dietary supplements. B is the largest and most structurally complicated vitamin and can be produced industrially through bacterial fermentation synthesis, typically used to manufacture B for fortified foods and supplements.  It can also be produced synthetically via vitamin B total synthesis.

David Sumner: David P. Sumner is an American mathematician known for his research in graph theory.  He formulated Sumner's conjecture that tournaments are universal graphs for polytrees in 1971, and showed in 1974 that all claw-free graphs with an even number of vertices have perfect matchings.

David P. Demarest: David P. Demarest (November 9, 1931 – October 15, 2011) was an American academic and writer best known for his work on organized labor, social geography, and US working-class literature.

David Bartel: David P. Bartel, Ph.D. is an American molecular biologist best known for his work on microRNAs. Bartel is a Professor of Biology at the Massachusetts Institute of Technology, Member of the Whitehead Institute, and investigator of the Howard Hughes Medical Institute (HHMI).  He earned his B.A. in Biology from Goshen College and his Ph.D. in Virology from Harvard University under the mentorship of Jack W. Szostak.

Riverside Cemetery (Oswego, New York): Riverside Cemetery is a historic rural cemetery and national historic district located at Oswego in Oswego County, New York.  It was established in 1855 and designed by landscape architect Burton Arnold Thomas (1808-1880).  Within the boundaries of this contributing site are six contributing buildings, 12 contributing structures, and 18 contributing objects.  Notable burials include De Witt Clinton Littlejohn (1818-1892), Luther W. Mott (1874-1923), Newton W. Nutting (1840-1889), Joel Turrill (1794-1859), David P. Brewster (1801-1876), Rudolph Bunner (1779-1837), James Cochran (1769-1848), John C. Churchill (1821-1905), Leander Babcock (1811-1864), Abraham P. Grant (1804-1871), and Orville Robinson (1801-1882).

Odd Nosdam: David P. Madson (born 1976), better known by his stage name Odd Nosdam, is an American underground hip hop producer, DJ and visual artist.  He is co-founder of the record label Anticon.  He has remixed tracks by a variety of bands and artists including Boards of Canada, The Notwist, and Sole.

David P. Robbins Prize: The David P. Robbins Prize for papers reporting novel research in algebra, combinatorics, or discrete mathematics is awarded both by the American Mathematical Society (AMS) and by the Mathematical Association of America (MAA).  The AMS award recognizes papers with a significant experimental component on a topic which is broadly accessible which provide a simple statement of the problem and clear exposition of the work.  Papers eligible for the MAA award are judged on quality of research, clarity of exposition, and accessibility to undergraduates.  Both awards consist of $5000 and are awarded once every three years.  They are named in the honor of David P. Robbins and were established in 2005 by the members of his family.

David Ballou: David P. Ballou is a professor of biological chemistry at the University of Michigan Medical School in the United States.  He is best known for his development of rapid-reaction techniques, including stopped flow and rapid freeze-quench EPR methods, as tools to study the mechanisms of enzymes containing flavin, iron, cobalamin, or pyridoxal phosphate cofactors.  Many of these studies were performed in collaboration with other scientists, most often with colleagues at Michigan.

Boys' Latin of Philadelphia Charter School: Boys' Latin of Philadelphia Charter School is a Pennsylvania approved secondary charter school located in Philadelphia, Pennsylvania.  Boys' Latin was created by David P. Hardy (CEO) and Janine Yass (founder).  It was formerly known as Southwest Philadelphia Academy for Boys but was changed to Boys' Latin of Philadelphia prior to receiving its charter.

Elk (sculpture): Elk, also known as the David P. Thompson Fountain, David P. Thompson Monument, Elk Fountain, or the Thompson Elk, is an historic outdoor fountain and bronze sculpture by American artist Roland Hinton Perry.  The sculpture was donated to the city of Portland, Oregon, United States in 1900.  It is located in the Plaza Blocks in downtown Portland.

Supporting paragraphs: David Ballou Vitamin B12
Answer: Vitamin B