4. Let {X1, X2, ..., Xn} be an independent positive random sample from a population with density function given by f(x; 0) = coz exp(-Bx) for x > 0, where the unknown parameter vector 0 = (a, B) with a E R and B > 0, and co > 0 is a constant only depending on 0. (i) Find a sufficient and complete statistic for 0. [4 marks] (ii) If E(X1) = 28 and E(log X1) = 2a + 3, find the UMVUE of 0. [6 marks] [Total: 10 marks]