The single most effective agent of chemical weathering at Earth's surface is

When china's civil war resumed, the nationalists held an advantage because the nationalist had more soldiers  than the communist.

Large asteroid and comet impacts. because they can have a huge impact on the environment, such as a loss of land, and crust on the earth with the big holes they produce. on the other hand, volcanic eruptions can also cause change within the environment by producing land, but im actually not quite sure which one would be a better answer, hopefully you read this and didnt just flat out choose the first sentence i have spoken! your welcome~