Researchers conducted an experiment to identify the effects of three different hand sanitizers on the growth of bacteria. The researchers grew one type of bacteria in twenty-four identical covered petri dishes. Then, they opened eighteen petri dishes and applied the three hand sanitizers to six petri dishes each. They left the remaining six petri dishes closed as a control group. When the researchers measured the growth of bacteria two days later, the six dishes that had no hand sanitizer showed the least amount of bacterial growth. The researchers concluded that not using any hand sanitizer would lead to the least bacterial growth.

Which statement best explains why the researchers’ conclusion was not justified?
The experiment tested too many variables.
The control group was not treated the same as the experimental group.
The sample size of twenty-four petri dishes is not large enough.
The single type of bacteria is not adequate to make any conclusions.

A scientific experiment is performed to test the predicted hypothesis which in the given case is that "different hand sanitizes have a different effect on the growth of the bacteria".

The researcher divides 24 samples into two groups- 18 Petri-dishes divided into 3 groups with 6 Petri dish receiving different sanitizers each and rest 6 as control samples without treatment of the hand sanitizers.

The researcher performed the mistake while doing the experiment as he opened the experimental Petri dishes and kept them open whereas in the control group he kept the Petri dish closed which lead to the error in the results as the control showed least growth of the bacteria which should be high.

This happened because the control and the experimental samples were not provided with the same question and thus is the correct answer.