A teacher's students were asking for individual assistance at a high rate when they were...

A teacher's students were asking for individual assistance at a high rate when they were instructed to work independently using their textbooks and other resource material available in the classroom. The teacher posted a "No Questions" sign and systematically ignored the students' questions when the sign was up. The rate of question-asking decreased to zero. Now the teacher simply puts up the sign whenever the students are to work independently and removes it when the students can ask questions. With respect to asking questions, the sign functions as: