Several studies have been conducted to link smoking with cancer, but none has been able to explain why some people who are regular smokers have never contracted cancer while some other who never smoked do.

• A. none has been able to explain why some people who are regular smokers have never contracted cancer while some others who never smoked do.
• B. none have been able to explain why some people who are regularly smoking have never contracted cancer while some other who never smoked have.
• C. none has been able to explain why some people who are regular smokers have never contacted cancer while some other who never smoked have.
• D. none have been able to explain why some people who are regular smokers have never contracted cancer while some others who never smoked have.