`A` is a set containing `n` different elements. A subset `P` of `A` is chosen. The set `A` is reconstructed by replacing the elements of `P`. A subset `Q` of `A` is again chosen. The number of ways of choosing `P` and `Q` so that `PnnQ` contains exactly two elements is a. `.^n C_3xx2^n` b. `.^n C_2xx3^(n-2)` c. `3^(n-1)` d. none of these