In DES (Data Encryption Standard) every time a block is encrypted the user requires to supply 16 keys comprised of 56 bits each. Mark the correct answer below: a) The rounds in DES, each uses a sub-key derived from the single key provided by the user. b) No, 16 plus 2, because of the transposition stage at the beginning and at the end. c) No, every 16 blocks it is necessary to use a different key to keep the entropy in order. d) No! 3 key bits are used for each round, and four for each transposition, so together 3*16+4+4=56, which is the size of the DES key.