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Question:

A scientist designs an experiment to test the hypothesis that exposure to more sunlight will increase the growth rate of elodea, a type of aquatic plant. The scientist has accumulated data from previous experiments that identify the average growth rate of elodea exposed to natural sunlight in the wild.
In the experiment setup, there are three tanks housing ten elodea each. Tank A is positioned in front of a window to receive natural sunlight similar to what elodea are exposed to; tank B is positioned in front of the same window but has an additional sunlight-replicating lamp affixed to it; and tank C is positioned in a dark corner with no exposure to natural sunlight.
When setting up the above experiment, the scientist has the option of using a separate water filter for each of the three tanks or using a single filtration system that attaches to all three and affects them simultaneously. Which of the following filter setups makes a more valid experiment and why?

A one filtration system for all three tanks, because this makes filtration a controlled variable
explanation

Using one filtration system for all three tanks keeps the water quality across all three tanks constant and eliminates experimental bias for this variable.