Please explain (short essay)

Assume our hypothetical law firm is representing a defendant in a personal injury claim involving a vehicular accident. The complaint that we are answering, in paragraph 8, alleges that our client was negligent. Our strategy in the case includes the plan to advance three different defenses: the defendant was not negligent; even if the defendant was negligent, plaintiff was also negligent and plaintiff's negligence contributed to the accident; and plaintiff was not wearing a seatbelt in his vehicle when the accident happened and the failure to wear a seatbelt contributed to plaintiff's injuries. For each defense, state whether the defense must be set forth in our answer as an affirmative defense and why