An electronics company just finished designing a new tablet computer and is interested in estimating its battery-life. A random sample of 20 laptops with a full charge was tested and the battery-life was found to be approximately normal with a mean of 6 hours and a sample standard deviation of 1.5 hours. Which of the following is the correct form for a 99% confidence interval?

a) .99( ) 6 2.576(0.3354) CI  =
b) .99( ) 6 2.576(1.5)CI  =
c) .99( ) 6 2.861(0.3354)CI  =
d) .99( ) 6 2.861(1.5) CI  =