The number of telephone calls N between two cities A and B varies directly as the population P_A, P_B respectively and inversely as the square of the distance D between A and B. Which of the following equations represents this relation?

• A.N = \(\frac{kp_A}{D^2} + {cp_B}{D^2}\)
• B.N = \(\frac{kp_AP_B}{D^2}\)
• C.N = \(\frac{kD_AP_D}{B^2}\)
• D.N = \(\frac{kD^2_AP_D}{B}\)
• E.