A．Provide extra energy to start the reaction.
B．Raise the temperature of the chemicals.
C．Release a chemical needed to start the reaction.
D．Lower the amount of energy needed to start the reaction.
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.