A．To show that enzymes are very effective.
B．To point out that enzymes can sometimes fail to work.
C．To explain what enzymes are made of.
D．To describe different types of enzymes.
A．The relationship between painting and sculpture.
B．The ideas behind an art ist' s work.
C．The practical value of a work of art.
D．The way the eye perceives shape in sculpture.
A．It is often displayed outdoors.
B．It does not always represent an object.
C．It is three-dimensional.
D．It is done by relatively few artists.