Fact 1: Hearing is used for sensing sound by animals.

Fact 2: All sound is caused by vibrations.

Given the two facts above, answer the question "Hearing is used for sensing _ by animals?" with the following options: 
- A Greek letter
 - waves
 - vibrations
 - motion
 - messengers
 - humans
 - four limbs
 - some mammals


 

   vibrations      