Rather than just answer A or B, here's how you can figure it out. This works on almost any carb.
Move the choke linkage so that the choke plate is completely open (vertical).
Now operate the throttle linkage and watch what is going on.
You'll see that one of the screws will be up against a step in the linkage when the throttle is closed. This is the idle speed screw. What it does is limit how far the throttle closes when you take your foot off the gas. When you turn that screw in, it's like stepping on the gas a little bit.
Now pull on the choke linkage so that the choke plate closes. Now you will see that a different screw moves in and takes over limiting how far the throttle closes. That is the fast idle speed screw. That step in the linkage won't go all the way to the idle speed screw because the fast idle screw has stepped in to take over.
Once you understand how it works, you'll always remember it and never need to ask.