Which of the following statements regarding "Six Sigma" is true?

?a $.$ ?

The term has two distinct meanings $$ one is statistical; the other is a comprehensive quality system.

? b $.$ ?

Six Sigma means that about $94$ ?percent of a firm's output is free of defects.

? c $.$ ?

The Six Sigma program was developed by Toyota in the $1970\text{'}$ s $.$

? d $.$ ?

The Six Sigma program is for manufacturing firms, and is not applicable to services.

? e $.$ ?

Six Sigma certification is granted by the International Standards Organization $($ ISO $).$