1 . Consider strings of length $99$, in which each character is one of the characters $a,b,c,d,e$. How many such strings have exactly $9$ letters $e$?
(a)
$\binom{99}{9}\cdot 4^{90}$
(b)
$5^{9}\cdot 4^{90}$
(c)
$\binom{99}{9}\cdot 5^{90}$
(d)
$\binom{99}{5}\cdot 5^{90}$
(e)
$\binom{99}{5}\cdot 4^{90}$