Suppose I use a monoalphabetic substitution cipher $($MSC$)$ for which the plaintext alphabet is the set of English characters and the ciphertext alphabet is a subset of the possible $2-$digit numbers. After encrypting the following plaintext:

I love a good code

I get the following ciphertext:

$172983265952708336794383497959$

Which of the following statements is true? $($Hint: try filling out the key starting at the beginning of the plaintext$/$ciphertext and moving to the right, and see if you can find a solution that accords with one of the answers below.$)$

Group of answer choices

I could have used an ordinary MSC $($with no enhancements$)$

My MSC must be augmented with a null, but no dowbleth required

My MSC must be augmented with a dowbleth, but no null required

My MSC must be augmented with a dowbleth and a null

I couldn't have used an MSC even with a dowbleth and$/$or a null