1 .
What does $2^{n} - 1 - n - {n \choose 2}$ count?
(a)
The number of subsets of a set of size $n$ that have size at least two.
(b)
The number of subsets of a set of size $n$ that have size at least three.
(c)
The number of bitstrings of length $n$ that have at most two 1's.
(d)
The number of bitstrings of length $n$ that have at least two 1's.