- A class of something means a type of something
- Classifying is when one sorts something by kind / type
- Hence, classifying is when one sorts something by class

- Classifying is when one sorts something by class
- Class is a measure of how a rock is formed
- So, rocks can be classified based on how they were formed

- Rocks can be classified based on how they were formed
- Igneous rocks are formed by magma cooling / crystallizing
- Therefore, a rock can be classified as igneous if it was formed from magma

Therefore, a rock can be classified as igneous if it was formed from magma
Question
In order for rocks to be classified as igneous, the rocks must

Context
- A class of something means a type of something 
- Classifying is when one sorts something by kind / type 
- Class is a measure of how a rock is formed 
- Igneous rocks are formed by magma cooling / crystallizing