Problem $3(20$ pts$)x_{n}dots{x}_n$ be iid $N(,{}^2).$ Consider the estimation of ${}^2.$

Which estimator achieves Oramer$-$Rao Lower Bound with finite sample size? Justify your answer by considexing both the cases of known $$ and unknown $.$

Which estimatos has the property that the asymptotic variance of it achieves the CramerRao Lower Bound? Jugity your answer by considering both the cases of known $$ and unknown $.$