Which of the Following Is the Best Description of Global

Which of the following is the best description of global warming?

A) A buildup of greenhouse gasses like carbon dioxide caused by emissions, which raises the global mean temperature of the earth, increasingly affecting its natural systems
B) A natural process of warming and cooling of the earth's atmosphere and hydrosphere that occurs in predictable and finite cycles every 25,000 years or so
C) A widely reported phenomenon that few scientists understand, leading to wild speculations and doomsday scenarios involving the collapse of the climate