Please explain the answers especially number 3. Thank you very much

3. Suppose you have a language with only 3 letters a, b, c, and they occur with frequencies 0.7, 0.2, 0.1, respectively. The following ciphertext was encrypted by the Vigenere method: ABCBABBBAC. Suppose you intercept this message and want to try to break it. Suppose you know that the key length is either 1, 2, or 3. Do the following: (a) Determine the most probable length of the key from among the choices 1, 2, or 3. Show how you derived it Once you have the most probable key length, determine the most probable key (b) utilizing the frequency information given in the problem. 4. Use the following Playfair matrix: I/J Encrypt the following messages: (a) (b) (c) "Help is on the way." "Urgent. Respond at Once." "SOS" 5. Generate the Playfair matrix that corresponds to the keyword "secret". 6. Use the Playfair matrix from number 5, to encrypt the three messages from number 4 parts a, b, and c