For Statistical Inference About the Mean of a Single Population

For statistical inference about the mean of a single population when the population standard deviation is unknown,the degrees for freedom for the t-distribution equal n -1 because we lose one degree of freedom by using the:

A) sample mean as an estimate of the population mean.
B) sample standard deviation as an estimate of the population standard deviation.
C) sample proportion as an estimate of the population proportion.
D) sample size as an estimate of the population size.