Which of the following ports must be open (e.g. not blocked by a firewall) to allow Remote Assistance?

A technician needs to connect to a users desktop using Remote Assistance but is unable to connect and the invitation keeps timing out. Which of the following ports must be open (e.g. not blocked by a firewall) to allow Remote Assistance?

A.
TCP port 21

B.
TCP port 3389

C.
TCP port 80

D.
TCP port 3359