When constructing a confidence interval for a population mean from a sample of size 21, what is the number of degrees of freedom and for the critical t-value?

a) Degrees of freedom = 20, Critical t-value = 1.725
b) Degrees of freedom = 20, Critical t-value = 2.086
c) Degrees of freedom = 19, Critical t-value = 2.080
d) Degrees of freedom = 19, Critical t-value = 2.086