The figure shows a student's production function with the final grade (the output) related to the number of hours spent studying (the input). Which of the following statements regarding marginal product of labour (MPL) are correct? Select all the correct answers. 100 90 80 Final grade 6 8 8 8 68 8 O 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Hours of study per day Study hours 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 or more Grade 0 20 33 42 50 57 63 69 74 78 81 84 86 88 89 90 O The MPL is the slope of the tangent line.