Answer the following question given this paragraph: 

Skeletons made of cartilage rather than bone. Cartilage is supportive tissue that does not have as much calcium as bones, which makes bones rigid. Cartilage is softer and more flexible than bone.


Q: What does cartilage lack compared to bones, making it softer and less rigid?

 Choices:

- potassium

- magnesium

- protein

- calcium

A:
calcium
