An ammunition producer claims his best product has an average lifespan of exactly 18 years. A skeptical product evaluator asks for evidence (data) that might be used to evaluate this claim. The product evaluator was provided data collected from a random sample of 50 people who used the product. Using the data, an average product lifespan of 15 years and a standard deviation of 8 years was calculated. Select the 90%, confidence interval for the true mean lifespan of this product.