If n and r are nonnegative integers with r ≤ n, then the relation between n choose r ( n ) and ( n ) ( r ) (n r) is __________.
a. n choose r = ( n ) / ( r ) * (n - r)
b. n choose r = ( n ) * ( n - r) / ( r )
c. n choose r = ( n ) / ( r ) / (n - r)
d. n choose r = ( n ) * (n - r) / ( r )