B, diabetes is caused by insufficient insulin secretion, which can be treated by injecting insulin, so it is not the main factor threatening human health.
C cardiovascular diseases refer to arteriosclerosis, hypertension, coronary heart disease, myocarditis, congenital heart disease, etc. These diseases are difficult to cure; Cancer cannot be cured, so it meets the problem.
D. Hereditary diseases refer to diseases caused by genetic material changes or controlled by pathogenic genes. Hereditary diseases refer to diseases that are completely or partially determined by genetic factors, often congenital or acquired. Such as congenital stupidity, polydactyly, congenital deafness, hemophilia, etc. These hereditary diseases are completely determined by genetic factors, which appear at a certain time after birth, and sometimes it takes several years, ten years or even decades to become obvious.
So choose: C.