23（）

单项选择题According to the passage, a barter economy can lead to （）

A．rapid speed of transaction
B．misunderstanding
C．inflation
D．difficulties for the traders

单项选择题The possibility that a virus causes human cancer is indicated by （）

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