High cholesterol levels are highly correlated with cardiovascular disease. In the Italian town of Limone, however, each of the residents has had high cholesterol levels for many years, and yet they have not developed cardiovascular disease. Which one of the following, if true, most helps to explain the unusual health condition of the residents of Limone? ['The residents of Limone inherited from common ancestors a blood protein that prevents vascular blockage, which is a cause of cardiovascular disease.', 'Olive oil is a staple of the diet in some parts of Italy, but unlike butter, olive oil is a monosaturated fat, and monosaturated fats do not contribute to cardiovascular disease.', 'The stress and pollution found in large cities exacerbate existing cardiovascular disease, but there is little stress or pollution in Limone.', 'The residents of Limone have normal blood sugar levels, and very low blood sugar levels tend to cancel out the cardiovascular effects of a high cholesterol level.']