(Your answer here will obviously depend on your individual situation. Here are two scripted responses. The first is for people who majored in a subject or area that directly applies to their careers. The second is for those who majored in one thing and are doing another.)
I always knew ____________________ (business management/electrical engineering) was where I had the most potential, and I’ve remained with it because I turned out to be right. Not everyone is as fortunate as I have been. It’s very difficult at 18 to predict and plan for the future; but, even then, I knew what I wanted to do. I’m glad I set my goals at a young age. It has worked out well for me.
OR
When I was 18, nothing in the world seemed so important as ______________ (U.S. history/philosophy/English literature), and I studied for the sake of learning. Later, I realized I needed further education in subjects that would help me in my career.