Q:Fact 1: Carbon dioxide concentrations in the air have increased over the last decade dramatically. Fact 2: Carbon dioxide, CO2, is usually a harmless gas. Given the two facts above, answer the question "what concentration in the air has increased dramatically over the last decade?" with the following options: - zinc - CO2 - h2o - CFCs - lungs - coal - O2 - CO 2
A:
CO2