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 the average product of labour $($APL$)$ is correct?

$\backslash$table$[[$Study hours,$0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15$ or more$],[$Grade$,0,20,33,42,50,57,63,69,74,78,81,84,86,88,89,90]]$

The APL is decreasing beyond $15$ hours of study.

The APL at $10$ hours is $3.$

The APL is the slope of the tangent line.

The APL at $0$ hours is $20.$