Keshan disease is an endemic cardiomyopathy of unknown causes. Both epidemics of Keshan disease in Shandong Province occurred during the difficult life of the residents in the ward. At this time, residents had a single diet and lack of nutrition. After 1978, due to the rural economic system According to the reforms, the living conditions of the residents in the ward gradually improved, the nutritional structure became more reasonable, and fewer and fewer Keshan disease patients reached the basic control standard. Based on this, some researchers speculated that nutritional deficiency may be an important factor in the development of Keshan disease. If the following are true, which one cannot question the above inference? ['The soil and water quality of the original Keshan disease area has changed greatly in decades.', 'Prior to 1978, rural living standards were generally low, but Keshan disease only appeared in some places (partially acknowledged the relationship between nutritional deficiencies and Keshan disease, cannot be questioned), partly acknowledged the inference, and could not be questioned.', 'Keshan disease cases have also occurred in some areas of high living standards.', 'Keshan disease cannot be cured by adjusting the diet.']