In research from work on Alzheimer's disease and the prion diseases such as mad cow disease, the formation of disease causing fibrils in the brain is believed to result from

A. covalent crosslinking between fibrous proteins

B. first the conversion of alpha helical structures to beta sheets then the formation of large aggregates

C. first the conversion of beta sheet structures to alpha helices then the formation of large aggregates

D. triple helices wrapped together and then joined by covalent crosslinks from two uncommon amino acids.