Answer: A. 21
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C(n, r) = (n!)/(r!*(n-r)!) is the combination formula
C(7, 5) = (7!)/(5!*(7-5)!)
C(7, 5) = (7!)/(5!*2!)
C(7, 5) = (7*6*5!)/(5!*2!)
C(7, 5) = (7*6)/(2!) ..... note the "5!" terms divided and canceled
C(7, 5) = (7*6)/(2*1)
C(7, 5) = 42/2
C(7, 5) = 21
