For conducting an inferential study on the mean serum potassium concentrations in healthy women, a sample of 84 women was considered and a t-test was performed. The goal was to check if the mean serum potassium concentration is 4.2 mEq/l or not. What is the degree of freedom for the t-test?
(a) 168,
(b) 166,
(c) 84,
(d) 83