Students in an ecology class measured the length and surface area of red oak leaves collected on campus. The leaves were chosen based on their position on the tree: sun leaves were located on the outside edge of the tree, and shade leaves were located deep inside the canopy of the tree. A sample of 12 shade leaves had a mean length of 12.6 cm, with a standard deviation of 2.37 cm. Calculate a 95% confidence interval for the population mean for length of leaves in the shade. Which is the correct critical value of t?