7) The students' demand for Econ 101 Textbooks is represented by the function Q = 60 2P. Because the textbooks can be shared with other students, it creates a positive externality. The social equilibrium quantity for textbooks is 70. Each textbook costs $10. What is the value of the positive externaty? a) 90 b) 45 c) 30 d) 15 e) O f) -30