Which one of the following statements about influence of smoking on risk of coronary heart disease(CHD) is not true?

• A. Influence of smoking is only additive to other risk factors for CHD.
• B. Influence of smoking is independent of their risk factors for CHD.
• C. Influence of smoking is directly related to number of cigarettes smoked per day.
• D. Influence of smoking is synergistic to other risk factors for CHD.