The mean gross annual incomes of certified welders are normally distributed with the mean of $20,000 and a standard deviation of $2,000. The ship building association wishes to find out whether their welders earn more or less than $20,000 annually. The alternate hypothesis is that the mean is not $20,000. The alternate hypothesis is that the mean is not $20,000. If the level of significance is 0.10, what is the critical value? a. 1.65
b. 1.28
c. ±1.28
d. ±1.65