Faris puts a sign on his door. When the sign is flipped to t…
Faris puts a sign on his door. When the sign is flipped to the red side, he will not answer the door if a student knocks. When the sign is flipped to the purple side, he will answer the door if a student knocks. His students do not knock on the door when the sign is red. Faris has used _________ to ensure his students do not knock on his door when he is not available.