In the early 20th century the study of rocket propulsion was promising. which word or phrase is the subject complement in the sentence

Why is it important to be aware of you audience's interests? a. so that you can teach them new information thy didn't know b. so that you can use opposite ideas in order to shock them c. so that you can use the most effective strategy to persuade them d. so that you can pretend to like the same things and get them to trust you