A Six Sigma level is calculated by which of the following equations:

Select one:

a$.($Opportunities $-$ Defects$)/$Opportunities x $100$

b$.($Opportunities $+$ Defects$)/$Opportunities x $100$

c$.($Defects $-$ Opportunities$)/$Defects x $100$

d$.($Defects $+$ Opportunities$)/$Defects x $100$