Let A={1,2}, B={2,4}, C={4,5,6}. Which of the following may be considered as the universal set for set A, B, C?

• A. {1,6,7,8,9}
• B. {1,2,3,4}
• C. {2,4,5,6}
• D. {1,2,3,4,5,6}