单项选择题
A．was the first reliable experiment in language study in history B．proved what it meant to prove C．was not necessarily sound though based on observation D．was utterly ridiculous and not worth mentioning
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.