For the question four in the midterm, the most important point is to remember and understand every test of the convergence of the series clearly. Based on the theory, the second step is to link (a) and (b) together can connect to the (c). The tricky part of this question is you can treat the graph of (a) and (b) as the front elevation and the vertical elevation of a cuboid. They have a different length side, but also have a same length side which connect them.