The gray balls represent and have a neutral charge.

Neutrons might be the answer

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.

i think it is true since you can test the answer and can use observations to complete it.