This is the main human activity that contributes to global warming ...

A-massive dumping of pesticides into the worlds oceans

B-high demand and depletion of the worlds oil supply

C-burning fossil fuels

D-increase of levels of chlorine poured into the shorelines

I believe the answer would be C, burning fossil fuels. Oil and gas is made up of the remains of microscopic plankton. Over millions of years the remains become the carbon-rich coal, oil and gas we can use as fuel. When fossil fuels are burned they release carbon dioxide into the atmosphere, thus contributes to global warming.