Six books are kept one on top of the other. B1 and B2 are always kept together. Three books are kept above B5. B6 is neither placed at the top nor at the bottom. B1 and B2 cannot be placed with B5. Which book is kept at the bottom?
1
B1 or B3
2
B2 or B3
3
B3 or B4
4
B1 or B4