Consider the Following Diagram, and Then Answer the Question Below

Multiple Choice

Consider the following diagram, and then answer the question below.

-According to current scientific consensus, what is the reason for the upward trend in overall CO₂ concentrations demonstrated in the graph above?

A) changes in Earth's atmosphere brought about by natural climate variation
B) large and increasingly frequent volcanic eruptions
C) rapidly increasing fossil fuel combustion and the resulting emissions
D) The causes are still unknown.