Which describes the independent variable in an experiment? the variable changed by the experimenter the quantity that must remain the same the variable that changes in response to the experiment the statement that is worded to test the experiment

Astudent poured a solution of bromothymol blue indicator into three test tubes. then he placed an aquatic plant in two of the test tubes, as shown below. he placed a stopper on each test tube and placed them all in the dark for 24 hours. bromothymol blue turns from blue to yellow in the presence of co2