At the beginning of the semester, the humanities instructor explains that there will be ten surprise quizzes over the semester, so students are strongly encouraged to keep up with their reading and attend every class. the humanities instructor is using a schedule of reinforcement to encourage studying and class attendance.

variable - interval

Explanation: In the this scenario, The humanities instructor is using a variable-interval schedule of reinforcement. The instructor used the word 'surprise' meaning the time and date of the test isn't fixed and the quizzes will come up at unpredictable time intervals. This kind of reinforcement results in a steady response rate as it keeps students on their toes simply due to the fact that they have no idea of when the quizzes are coming up.