There Are Special Ring Tones for Cell Phones That Can't

There are special ring tones for cell phones that can't be heard by adults. Which of the following makes that possible?

A) Younger adults are more sensitive to lower-pitch tones than older adults.
B) Older adults are more sensitive to higher-pitch tones than younger adults.
C) Younger adults are more sensitive to higher-pitch tones than older adults.
D) Older adults are more sensitive to lower-pitch tones than younger adults.