Correct Answer : Option (B) :   It allows it to varies from one panel to next panel
                                                                                                                
                                                                                                                                                                        
                                                        Explanation : The source strength ‘X’ per unit length be constant over a given panel but allow it to vary from one panel to the next, that if there are total n panel. The source panel strength per unit length is ‘Xn’. Therefore boundary condition is imposed numerically by defining the midpoint of each panel.