2. Artificial potential field (APF) is a popularly used method for obstacle avoidance. Answer the following questions about APF. (Marks: 15%) (a) What is the principle of APF to avoid obstacle and simultaneously move to the target (4%)? (b) When there are more than one obstacles, how to avoid the collision with any of them (4%)? (C) What is deadlock in APF (4%)? (d) How to address deadlock (3%)