A sample of 14 joint specimens of a particular type gave a sample mean proportional limit stress of 8.59 MPa and a sample standard deviation of 0.72 MPa. (a) Calculate and interpret a 95% lower confidence bound for the true average proportional limit stress of all such joints. (Round your answer to two decimal places.) MPa Interpret this bound. With 95% confidence, we can say that the value of the true mean proportional limit stress of all such joints is less than this value. With 95% confidence, we can say that the value of the true mean proportional limit stress of all such joints is greater than this value. With 95% confidence, we can say that the value of the true mean proportional limit stress of all such joints is centered around this value. What, if any, assumptions did you make about the distribution of proportional limit stress