Calculate the smallest sample size required to estimate the population mean under the following specifications:
a. confidence level 95%, σ = 16, and e = 4
b. confidence level 90%, σ = 23, and e = 0.5
c. confidence level 99%, σ = 0.5, and e = 1
d. confidence level 98%, σ = 1.5, and e = 0.2
e. confidence level 95%, σ = 6, and e = 2