Suppose I use the simple binary substitution cipher mentioned in the last problem to encrypt a secret message, represented in binary ASCII representation. As a result of encryption, I$\text{'}$m left with nothing but a string of straight $0\text{'}$s$.$ Which of the following must be true?

Group of answer choices

The only letter in my secret message is the lowercase $\text{'}$a$\text{'}.$

The secret message was itself a set of straight $0\text{'}$s$.$

The keyphrase was itself a sequence of straight $0\text{'}$s

The plaintext is identical to the keyphrase I used.

The fact that we got straight $0\text{'}$s doesn't tell us any of these things.