Why does dna contain thymine but rna contains uracil?

If the number of trees on earth continues to decrease significantly, what might be the result? a. less oxygen released into the atmosphere and more co2 buildup; erosion could occur without the tree roots to hold the earth in place; loss of habitat of many animals and diversity could decrease b. balance of nitrogen and phosphorus in the ecosystem and the movement of water through the ecosystem could be thrown out of balance c. a and b d. none of the above