Fact 1: Coal is used for generating heat by burning.

Fact 2: Coal and lignite are the lithified remains of plants.

Given the two facts above, answer the question "What are the lithified remains of plants used for?" with the following options: 
- heat production
 - generating heat by burning
 - harvesting crops
 - damming rivers
 - food and shelter
 - heat is produced
 - A transportation fuel
 - planting seeds


 

  generating heat by burning       