1 .
Consider strings of length 15, where each character is a lowercase letter or an uppercase letter.
How many such strings contain at least two lowercase letters?
(a)
$52^{15} - 15 \cdot 26^{15}$
(b)
$52^{15} - 26^{15} - 15 \cdot 26^{14}$
(c)
$52^{15} - 26^{15} - 15 \cdot 26^{15}$
(d)
None of the above.