Six sigma is $\_\_\_\_\_.$

an approach to quality improvement that calls for everyone within an organization to take responsibility for improving quality and emphasizes the need for a long$-$term commitment to continuous improvement

an approach to quality improvement characterized by very ambitious quality goals, extensive training of employees, and a long$-$term commitment to working on quality$-$related issues

a family of generic standards for quality management systems established by the International Organization for Standardization $($ISO$)$

a national program to encourage American firms to focus on quality improvement