Lower and Upper Sum of a Function

Here, you can see a function with its lower sum and upper sum on the interval [A, B].
1) Drag the endpoints A and B of the interval with the mouse. What characteristics of the rectangles of the (a) lower sum, (b) upper sum do you see? 2) Now, change the number of rectangles n by moving the slider with the mouse. How does this affect the difference between lower and upper sum?