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1 . Let $n \geq 2$ be an integer. How many bitstrings of length $n$ are there that contain at least two 1s?

(a)

$2^{n} - 1 - n$

(b)

$n \cdot 2^{n-1}$

(c)

${n \choose 2} \cdot 2^{n-2}$

(d)

$2^{n} - n$