443. A civil engineer is analyzing the compressive strength of concrete. Compressive strength is approximately normally distributed with variance o2 = 1000 psiz. A random sample of 12 specimens has a mean compressive strength of .7? = 3255.42 psi. (a) Test the hypothesis that mean compressive strength is 3500 psi. Use a xedlevel test with or = 0.01. (b) What is the smallest level of signicance at which you would be willing to reject the null hypothesis? (c) Construct a 95% twosided Cl on mean compressive strength. (d) Construct a 99% twosided Cl on mean compressive strength. Compare the width of this condence interval with the width of the one found in part (c). Comment