註釋
Cambridge Discovery Education Interactive Readers are the next generation of graded readers - captivating topics, high-impact video, and interactive exercises designed to motivate and engage. Why do we do good things for one another? Do we still do good things if we get nothing in return? This reader explores the science, impulses, and reasons behind altruism in both humans and animals. Use the unique code inside the book to access the online reader, complete with videos and interactive exercises. Word count: 3007. Lexile: 690L. 
