question: Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
  Question: Fig.Q3 shows an excerpt of the transmission phase of a TCP connection. Assume the length of the IP header is 20 bytes. What is the ACK number at message 6?
  Choices:
  (A) 839
  (B) 510
  (C) 623
  (D) 1024