A) coarse C) lesson
B) class D) text

A．curled
B．turned
C．folded
D．bent

A．To show that air behaves like water in some aspects.
B．To illustrate the special sequence of procedures involved in the establishment of the working laws of science.
C．To indicate that it was difficult to use scientific methods to solve the problem of vacuum pump.
D．To show how scientists make experiments to test their deductions.