Fact 1: Cartilage is a tough tissue that contains a protein called collagen.

Fact 2: Sharks have a cartilage skeleton.

Given the two facts above, answer the question "What has skeleton made of tough tissue that contains a protein called collagen?" with the following options: 
- mammals
 - rocks
 - animals
 - sharks
 - a fish
 - cows
 - humans
 - plants


 

    sharks     