You are having a debate with someone in a library. A librarian asks you to "speak softer." What characteristic of sound does the librarian want you to change and how can you change it?
a. wavelength, by lowering the amplitude at which you are speaking
b. amplitude, by lowering the frequency at which you are speaking
c. frequency, by lowering the volume at which you are speaking
d. amplitude, by lowering the volume at which you are speaking.