Can someone help me answer these parts of my physics problem. I could also use help with the extra credit

1. While saving the world one weekend, The Courageous Dr. Bob must tackle a I40 kg bad guy. Before the collision, the bad guy is running north at 3.0 m/s, and The 70. kg Courageous Dr. Bob is running west at 4.0 m/s. A. Why are we not going to use energy conservation to treat this opportunity? (5 points) B. Why are we not going to use Newton's 2nd Law directly (that is, by identifying the forces) to treat this opportunity? (5 points) C. Why are we going to use momentum conservation to treat this opportunity? (5 points) D. If the collision is completely inelastic, what will be the velocity of The Courageous Dr. Bob and the villain after the collision? (50 points) B. What is the change in kinetic energy? (25 points) F. Where did the \"missing\" energy go? (5 points) G. Estimate the weight of the bad guy in units of pounds. (5 points) Extra Credit: It is observed that adding 1.0kg of mass to a spring causes it to stretch .IOm. What is the spring's spring constant? (5 points)