The smaller the average pH value of soil is, the stronger the acidity is, so it can be seen from the figure that the pH value of stage C is the smallest, so the most serious stage of soil acidification in this area is stage C; The cause of soil acidification may be caused by the combustion of a large number of sulfur-containing coal, and calcium hydroxide is alkaline and can be used to improve acidic soil.
So, the answer is: c; Caused by the combustion of a large number of sulfur-containing coal; Apply calcium hydroxide to the soil to improve its acidity.