Consider different activities related to email.

s$1$: Send an email from a mail client to a mail server

s$2$: Download an email from mailbox server to a mail client

s$3$: Checking email in a web browser

$\_\_\_\_\_\_$ is the application level protocol used in each activity.

A$.$ s$1$ : HTTP $,$ s$2$ : SMTP $,$ s$3$ : POP

B$.$ s$1$ : SMTP $,$ s$2$ : FTP $,$ s$3$ : HTTP

C$.$ s$1$ : SMTP $,$ s$2$ : POP $,$ s$3$ : HTTP

D$.$ s$1$ : POP $,$ s$2$ : SMTP $,$ s$3$ : IMAP