That would be natural selection, artificial selection involves a human choosing for a beneficial trait
Answer and explanation
There are instances where taking any vitamin or mineral supplements will be unnecessary or even harmful.
Example
a) For children who are already drinking Fluoridated it will be unnecessary to give them Fluoride supplements
b) It is unnecessary to take supplements because you believe it can cure diseases. If you have a disease one should see a doctor
c) It can be harmful to take supplements that increase energy levels inthe body. It can lead to altered heart rhythm and eventually death
d)A doctor should be consulted before taking nutrient supplements such as Vitamin A. Most have a toxicity level and it is not necessary to take them unless there is deficiencies.
Your question is rather unclear but from what I can tell either there were not an even amount of Petri dishes or some of them were left open while others were not.
