The number of telephone call between two cities A and B varies directly as the population [insert mathematics] in A and 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 = ^kP_A/_D² + ^CP_B/_D²
• B.N = ^KP_APB_B/_D²
• C.N = KDP_AP_B
• D.Nota