单项选择题
Passage Three Questions 23 to 25 are based on the passage you have just heard.
According to the passage, why haven’t scientists found a cure for the common cold （）
A．They haven’t devoted its much energy to medicine as to space travel.
B．There are too many kinds of cold viruses for them to identify.
C．It is not economical to find a cure for each type of cold.
D．They believe people can recover without treatment.