There was a total of 21 math books in the two drawers of Mrs. Luca's classroom desk, including the 8 books she found in the bottom drawer. Which equation can be used to find b, the number of books that were in the top drawer?
A. b ÷ 8 = 21
B. b − 8 = 21
C. 8b = 21
D. b + 8 = 21

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