Problem $1.$ Nam$-$joo has just intercepted a long piece of Vign$$re ciphertext. She knows it is the encryption of

English plaintext, and based on an analysis of indices of coincidence, she thinks that the period is most likely $5.$

When she rewrites the ciphertext into a rectangle of width $5,$ she finds that the the most frequent letter in the first

column is A$.$ Given just this information, give an example of a word that might be the keyword that was used for

encryption. Explain briefly.