In a population, a random variable follows a normal distribution with an unknown mean and a standard deviation of 5. In a sample of 100 selected at random, a sample mean of 80 was obtained. Determine the confidence interval with a confidence level of 97% for the average population.
a. 80-2.17*5/10, 80+2.17*5/10
b. 80-2.17*5/10, 80+2.17*5/10
c. 120-1.96*5/100, 120+1.96*5/100
d. 120-1.96*5/100, 120+1.96*5/100