Adam has a monthly income of $20 that can be spent on books (B) and pencils (P). The price of a book is $5 and the price of a pencil is $0.50. Which of the following bundles of books and pencils lies ON Adam's budget line?
A. 2 books and 20 pencils
B. 1 book and 40 pencils
C. 3 books and 5 pencils
D. no books and 20 pencils