A box in a college bookstore contains books, and each book in the box is a history book, an english book or a science book. If 1/3 of these books are history books and 1/6 are english books, what fraction of these books are science books?
A1/4
B1/2
C2/3
D3/4
A1/4
B1/2
C2/3
D3/4