d1 is not an unbiased estimator of μ because its expected value is not equal to μ, which is 1. The sample mean d2 is an unbiased estimator of μ because as the sample size increases, the expected value of the sample mean approaches the population mean μ. d2 has a smaller variance than d1 because the variance of the sample mean decreases as the sample size increases, making it a more precise estimator of the population parameter.