The job the part does within organism is the

d.) circle motion requires a force to give it a centripetal acceleration.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

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.