1 . What does the summation $ \sum_{k=7}^{n} \binom{k-1}{6} $ count?
(a)
None of the above.
(b)
The numbre of subsets of ${1, 2, 3, ..., n}$ having size 6
(c)
The numbre of subsets of ${1, 2, 3, ..., n}$ having size 5
(d)
The numbre of subsets of ${1, 2, 3, ..., n}$ having size 7