4. Read the following sentence from the section "Are All Enzymes Proteins?" Most of the time students study enzymes, they are really studying protein-based
enzymes, since very little is known about how RNA can act as a catalyst.
The author uses the word "catalyst" to mean...
A. someone whose enthusiasm causes others to be more energetic
B. something that generates activity between two or more people
C. a person who launches an event or change
D. a substance that causes or speeds up a chemical reaction