The answer is A) Plagues wiped out the entire crop in a large region. This would result in the dramatic lowering of genetic diversity within the crop species. This as we know is very bad for an organism, as a lower genetic diversity can result in deleterious genes becoming more common, which can dramatically lower the fitness of a species. By storing genetic information for cop species in a gene bank, we can maintain the wide genetic diversity of important crop species, in case some sort of disaster happens to wipe out much of the crop population within a particular year.