Which one of the following statements about influence of smoking on risk of coronary heart disease (CHD) is not true?
park's textbook of preventive and social medicine 23rd edition . 

there is evidence that influence smoking not only independent of but also synergistic with other risk factors such as hypertension and elevated serum cholesterol.this means that the effect are more than additive.
Influence of smoking is independent of the risk factors for CHD
Influence of smoking is only additive to other factors for CHD
Influence of smoking is synergistic to other risk factors for CHD
Influence of smoking is directly related to number of cigarettes smoke per day