Which of the following statements is an example of a theme?

A.The story takes place in the future and warns about the dangers of technology.
B. The main character of the story is a ten-year old boy named Troy.
C. It is always best to tell the truth, even if it is difficult.
D. When faced with a challenge, two brothers realize the importance of cooperation.

D. When faced with a challenge, two brothers realize the importance of cooperation.