This passage mainly centers on______.

A．college students always look down upon craftsmen
B．schools and books are not the only way to knowledge
C．scientific discoveries are based on personal experience
D．discoveries are the most important source of knowledge

A．talk to a fisherman as Aristotle once attempted to
B．be patient in helping a craftsman with scientific term
C．learn a craftsman's experience by examining it carefully
D．accept everything that a craftsman teaches as valuable