Which of the following is a sentence fragment?

A. Checking his mail each day was always fun.

B. Knowing he would be late, she took a nap.

C. Realizing you are getting older is sobering.

D. Hiding in the tree fort he had built as a kid.

D. would be a sentence fragment because it dose not have a person or subject who is completing the action ... My teacher explains it as " If some one just came up to you and said 'Hiding in the tree fort he had built as a kid.' would you understand them? You would be so confused but if it had a subject 'He was hiding in the tree fort he had built as a kid.' It would still be strange but you would understand what they were saying."