Question: 2017 Winter Midterm - 6
Let $n \geq 4$ 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 $ab$ or start with $abc$?
(a)
$4^{n-1}$
(b)
$4^{n-2}$
(c)
$4^{n}$
(d)
$4^{n-3}$