A．fair
B．third
C．bird
D．shirt

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

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