A shoemaker claims his best product has an average lifespan of exactly 14 years. A skeptical customer asks for evidence (data) that might be used to evaluate this claim. The customer was provided data collected from a random sample of 40 people who used the product. Using the data, an average product lifespan of 21 years and a standard deviation of 4 years was calculated. Select the 99%, confidence interval for the true mean lifespan of this product.

a. [19.368, 22.632]
b. [-1.6317, 1.6317]
c. [20.742, 21.258]
d. [19.103, 22.897]
e. [12.368, 15.632]