The word "issue" in the last paragraph can be replaced as _________.

A．trees' function for absorbing carbon dioxide is limited
B．carbon dioxide is the main cause for global warming
C．leaves falling from trees can emit more oxygen
D．more carbon dioxide is beneficial for trees' growth for many decades

A．ice from Antarctica and Greenland is appropriate to show the change
B．carbon dioxide is released mainly from the fuel
C．level of carbon dioxide increases with global temperatures
D．carbon dioxide is helpful for tree growth