You are presented with the quiz "When is coal burning _ is given off" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "coal is used for generating heat by burning" and "cO 2 and SO 2 are both given off when coal is burned to provide heat or to generate electricity". 

So, what's the best answer to the question?


 

 CO 2          