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Question: 2015 Fall Final - 10

Author: Michiel Smid
What is the number of bitstrings of length $n$ that contain 00 or 11?
(a)
$2^n - 2$
(b)
$2^n - 4$
(c)
$2 \cdot n \cdot 2^{n-1}$
(d)
$2 \cdot (n-1) \cdot 2^{n-2}$