Students perform an experiment using an incompressible, ideal fluid of unknown density \(\rho\). The fluid is held in a wide horizontal reservoir at a constant absolute pressure \(P_0\) and discharges horizontally into the atmosphere at pressure \(P_{\text{atm}}\) through a circular exit nozzle of diameter \(d\), where \(P_0 > P_{\text{atm}}\). The speed of the fluid inside the reservoir is negligible compared to the exit speed. The students measure the volume flow rate \(Q\) for several different nozzle diameters while maintaining a constant pressure difference \(\Delta P = P_0 - P_{\text{atm}}\). Which of the following pairs of quantities should be plotted to produce a linear graph, and what is the theoretical slope of the best-fit line?