How do you write and present research well? 7—Cite to get cited

We cite other researchers to recognize their contribution, educate, and establish the foundation of the research. References should balance recent breakthroughs with past contributions. Between 2011 and 2014, Nature published over 3200 articles and letters that referenced 113 000 papers. What is the average age of the references in these papers (the difference between the year Nature published the paper and the year the cited article was published)?[1] 4 6 10 12