You are presented with the quiz "What has led to the increase in global warming?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "carbon dioxide concentrations in the air have increased over the last decade dramatically" and "carbon dioxide is the chief culprit in the increased threat of global warming in recent decades". 

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


 

   Increase in carbon dioxide        