Fact 1: Most mollusks have shells.

Fact 2: Clams, oysters and snails are familiar mollusks.

Given the two facts above, answer the question "what usually has a shell?" with the following options: 
- an object
 - humans
 - animals
 - density
 - tortoises
 - barnacles
 - h2o
 - oysters


 

        oysters 