1 .
Let $n \geq 5$ and consider strings of length $n$ consisting of the characters $a$, $b$, $c$, and $d$.
How many such strings are there that start with $ad$ or end with $dcb$?
(a)
$4^n - 4^{n - 5}$
(b)
$4^n - 4^{n - 2} - 4^{n - 3}$
(c)
$4^{n - 2} + 4^{n - 3} - 4^{n - 5}$
(d)
$4^{n - 2} + 4^{n - 3}$