What is the key space of the substitution cipher, as described in the text? $($Select all that apply$)$

Group of answer choices

The key space is determined by the number of letters in the alphabet.

The key space is the result of multiplying the number of possible replacements for each letter.

The key space is equal to the factorial of the number of letters in the alphabet $(26!$ in this case$).$

Testing all possible keys of the substitution cipher would take several decades even with hundreds of thousands of high$-$end PCs$.$

The key space of the substitution cipher is computationally secure against a brute$-$force attack.