n(n−r)r
nr(n−r)
1r(n−r)
n+1−rr
Correct answer is D
nCr=n!(n−r)!r!
nCr−1=n!(n−(r−1))!(r−1)!
nCr÷nCr−1=n!(n−r)!r!÷n!(n−(r−1))!(r−1)!
= n!(n−r)!r!×(n−(r−1)!(r−1)!n!
= (n+1−r)!(r−1)!(n−r)!r!
= (n+1−r)(n−r)!(r−1)!(n−r)!r(r−1)!
= n+1−rr
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