A manufacturer claims that the average tensile strength of thread A exceeds the average tensile strength of thread B by at least 12 kilograms. To test this claim. 50 pieces of each type of thread were tested under similar conditions. Type A thread had an average tensile strength of 86.? kilograms with a standard deviation of 6.28 kilograms. while type B thread had an average tensile strength of 77.8 kilograms with a standard deviation of 5.61 kilograms. Test the manufacturer's claim using a 0.05 level of signicance. What is the critical value? -1.66 +1.28 +1.66 1 2.00 i 1.96 -1 .28 O