Question : Let A be a set containing n elements. A subset P of the set A is chosen at random. The set A is reconstructed by replacing the elements of P, and another subset Q of A is chosen at random. The probability that PnQ contains exactly m (m<n) elements is
(A) \dfrac{3^{n-m}}{4^{n}}
(B) n_{C_{m}}\times \dfrac{3^{m}}{4^{n}}
(C) n_{C_{m}}\times \dfrac{3^{n-m}}{4^{n}}
(D) none of these
Answer : option (C)
Solution :
