An educator wanted to look at the study habits of university students. As part of the research, data was collected for three variables - the amount of time (in hours per week) spent studying, the amount of time (in hours per week) spent playing video games and the GPA - for a sample of 20 male university students. As part of the research, a 95% confidence interval for the average GPA of all male university students was calculated to be: (2.95, 3.10). Which of the following statements is true? (Can you please do it by hand and explain-Thanks)

A) In construction of the confidence interval, at-value with 20 degrees of freedom was used.
B) In construction of the confidence interval, a z-value with 20 degrees of freedom was used.
C) In construction of the confidence interval, at-value with 19 degrees of freedom was used.
D) In construction of the confidence interval, a z-value was used.