OPTIONS:
[1]. energy of moving objects;
[2]. mining gold;
[3]. Warm air moving into cold air;
[4]. burning fossil fuels;
[5]. acid rain;
[6]. melting ice caps;
[7]. energy usage;
[8]. single-celled organisms;
Given those answer options, answer the question: You are presented with the quiz "What is contributing to the greenhouse effect?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "burning fossil fuels increases the carbon in the atmosphere" and "this atmospheric carbon might be contributing to the greenhouse effect". So, what's the best answer to the question?
A:
[4].