How would I solve this?

We are trying to estimate the mean repair time / for aircraft engines. We have three different estimators that we can use: A, B, and C = A-4. A previous study has revealed that E[A] = u + 4, Var(A) = 12, E[B] = M, and Var(B) = 17. Which estimator has the lowest mean squared erro OA O B OC O Can't tell from the given information