填空题
A．the fact that viruses have been known to mutate B．the fact that a cancer-immune individual may lose his immunity C．the fact that production of human cancer cells might be due to a genetic factor D．the fact that man is host to many viruses
A．To outline contrasting types of economy. B．To explain the science of economics. C．To argue for the superiority of one economics. D．To compare barter and money exchange markets.