What is the function of a control group in an experiment? A. It is the group that always changes
B. To check the validity of an experiment
C. To test on the hypothesis of an experiment
D. It is the group that never changes

gravity causes stares to cluster i think so

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.