When Implementing the Vigenère Cipher, Which Step Comes First

Multiple Choice

When implementing the Vigenère cipher, which step comes first?

A) Use the accumulator pattern to add the ciphertext letter to the ciphertext message.
B) Determine which letter of the key we should use.
C) Return the string as the ciphertext message.
D) Look up the ciphertext letter in the Vigenère square, using the key letter row and plaintext character column.