A school library is planning to add a reference section to their collection. The library can add no more than 20 books, and the librarian decided that the school could use a maximum of 16 dictionaries and a maximum of 9 thesauruses. Dictionaries cost $15, and thesauruses cost $10. Write the objective function. Represent cost as C, dictionaries as x, and thesauruses as y.
a) C = 15x + 10y
b) C = 10x + 15y
c) C = 25x + 10y
d) C = 15x + 25y