An object moves with speed \( v_1 \), \( v_2 \), and \( v_3 \) along a line segment \( AB \), \( BC \), and \( CD \) respectively as shown in the figure. Where \( AB = BC \) and \( AD = 3 AB \), then the average speed of the object will be:
Which letter will come at the place of question mark (?)