10 books are placed at random in a shelf. The probability that a pair of books will always be together is_________?

A. 1/10
B. 9/10
C. 1/5
D. 3/10
Explanation:
10 books can be rearranged in 10! ways consider the two books taken as a pair then number of favourable ways of getting these two books together is 9! 2!
Required probability = 1/5