$[$Section: Introduction to Six Sigma: Defining Six Sigma$]$

A process that performs at a $6$ sigma level:

Select one:

a$.$ is considered statistically 'perfect'

b$.$ has $3\mathrm{.}4$ defects per million opportunities

c$.$ operates at a $99\mathrm{.}99966\%$ accuracy

d$.$ all of the above

e$.$ some of the above