What is an alloy? A. A metal with a changed oxidation state
B. a combination of a metal with another element
C. a layering of one metal over another metal
D. A substitution of one metal for another metal

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.

dynamic equilibrium is the answer.