The dual of a Boolean function F(x1, x2, … , xn, +, ∙ , ′), written as FD, is the same expression as that of F with + and ⋅ swapped. F is said to be self-dual if F = FD. The number of self-dual functions with n Boolean variables is

• A. 2n
• B. 2n-1
• C. \({2^{{2^n}}}\)
• D. \({2^{{2^{n - 1}}}}\)