HELP PLEASE!!!! I NEED THE ANSWER ASAP!

Why are many scientists concerned about humans adding large amounts of carbon to the atmosphere by burning fossil fuels (combustion)?
A.
The atmosphere has a limited capacity to absorb and recycle carbon naturally.
B.
Carbon is a poisonous chemical, which can kill plants.
C.
When carbon is released into the atmosphere it floats into outer space, and is lost forever.
D.
If the atmosphere gains too much carbon, birds won't be able to fly.