Answer:A.Preventing convection currents that take heat aloft, in the same way that the glass of a greenhouse stops convection currents and so makes the air in the greenhouse warmer.
Explanation: when ultraviolet solar radiation penetrates into the Earth's atomosphere and heats it,
the conventional radiation is supposed to be radiated back into the atmosphere as heat.however this is prevented from happening due to green house gases,they absorb the radiant heat and retain this heat within the Earth's environment and so behaves like the glass barrier in green houses.these green house gases trap heat within the Earth causing global warming .