What are the first and last terms of the following series ?

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.

1.solid

2.carbon dioxide

3.helium

im not exactly sure but i think its b!