The following functions can be pdfs if the constants are chosen properly. Find the proper conditions on the constants so that they are. [A, B, C, D, α, β, γ, and τ are positive constant and u (x) is the unit step function.]

(a) f(x) = Ae^-αx u(x), where u(x) is the unit step 

(b) f(x) = Be^βx u(-x)

(c) f(x) = Ce^-γx u(x - 1)

(d) f(x) = C [u(x) - u (x - τ)