Which of the Following Is the Best Tip for Replying

Which of the following is the best tip for replying to e-mail messages?

A) Reply to an e-mail message immediately because reading additional e-mail messages before replying might confuse you.
B) If you can't reply immediately, acknowledge receipt of the message and tell the sender when you'll be able to reply.
C) Include the sender's entire original message in the body of your reply to clarify the meaning of your response.
D) Retain the original subject line to identify a conversation thread even if the topic shifts.