Find the RaoCramr lower bound, and thus the asymptotic variance of the maximum likelihood estimator , if

Find the Rao–Cramér lower bound, and thus the asymptotic variance of the maximum likelihood estimator θ, if the random sample X1, X2, . . . , Xn is taken from each of the distributions having the following pdfs:
(a) f(x; θ) = (1/θ2) x e−x/θ, 0 < x < ∞, 0 < θ < ∞.
(b) f(x; θ) = (1/2θ3) x2 e−x/θ, 0 < x < ∞, 0 < θ < ∞.
(c) f(x; θ) = (1/θ) x(1−θ)/θ, 0 < x < 1, 0 < θ < ∞.