Each month, a nail manufacturer samples one thousand nails to make sure that they are the weight they are supposed to be, and reports this with a 95% confidence interval. About how often is the value from one month outside the interval reported the previous month?

Confidence Interval (%) 80 85 90 95 95.4 99 99.5 99.7 99.9
z-value 1.282 1.440 1.645 1.960 2 2.576 2.807 3 3.291

a) once every two years
b) once every three years
c) once every ten years
d) once every thirty years