If an estimator is unbiased, then A. the mean of the estimator is usually close to the true value. B. the estimator is usually close to the true value. C. the mean of the estimator is equal to the true value. D. the estimator is equal to the true value. The variance of an estimator measures A. how close the estimator is to the true value. B. how close repeated values of the mean of the estimator are to each other. C. how close repeated values of the estimator are to each other. D. how close the mean of the estimator is to the true value.