The library sells 500 books at its annual used-book fundraiser. They take in a total of $3940. Children's books cost $5, and adult books cost $9. How many children's books did they sell? Let e = number of children's books sold, and Let a= the number of adult books sold. Which system of equations can the librarians use to solve this problem? A. 5c+9a=500 c+a=3940 c+a=500 C. 5c+9a=3940 B. a=500-c 9c+5a=3940 a+c=500 D. 5a+9c=3940