1 .
Let $n \geq 7$ be an integer and consider strings of length $n$ consisting of the characters $a$, $b$,
$c$, and $d$. How many such strings are there that start with $abc$ or end with $dddd$?
(a)
$4^{n-3} + 4^{n-4}$
(b)
$2 \cdot 4^{n-3} - 4^{n-7}$
(c)
$2 \cdot 4^{n-4} - 4^{n-7}$
(d)
$4^{n-3} + 4^{n-4} - 4^{n-7}$