Read this excerpt from President Barack Obama's Back to School speech:

Now, I know it's not always easy to do well in school. I know a lot of you have challenges in your lives right now that can make it hard to focus on your schoolwork. I get it. I know what that's like. My father left my family when I was two years old, and I was raised by a single mom who had to work and who struggled at times to pay the bills and wasn't always able to give us the things that other kids had. There were times when I missed having a father in my life. There were times when I was lonely and I felt like I didn't fit in. What is the main idea of this excerpt?

A. Nothing easy is really worth doing, so students should feel grateful for their challenges. B. Students who face challenges outside of school should seek the help of a trusted adult. C. People who grow up without fathers have an excuse not to make an effort in school. D. Obama understands that personal problems can make it difficult to focus in school.