Question

stella wants to test the hypothesis \if carbon dioxide levels increase, the growth rate of sunflowers will increase.\ she places five young sunflower plants into similar containers and sets up pumps that release different amounts of carbon dioxide into each container. she plans to measure the daily height of the plants for two weeks and graph the data. which option would allow stella to set up a control group for the experiment? she should provide mist to each of the plants except one plant to keep the air moist and fertilize the plants. she should give the plants receiving the least co₂ a little more oxygen to compensate for the difference in gases. she should give all the flowers the same amount of water. she should place one flower in a similar container, but not pump carbon dioxide into it.

Explanation:

In an experiment, a control group is used to compare with the experimental groups. Here, the independent variable is carbon dioxide levels. The control group should have all other conditions the same as the experimental groups but with no change in the independent variable (carbon dioxide in this case).

• Option 1: Mist and fertilization are not related to the carbon - dioxide - growth - rate hypothesis and do not set up a proper control for carbon dioxide.
• Option 2: Adding more oxygen to compensate for less \(CO_2\) changes another variable (oxygen) and does not create a valid control for \(CO_2\).
• Option 3: Giving the same amount of water is a way to control a confounding variable (water), but it is not setting up a control group for the independent variable (\(CO_2\)).
• Option 4: Placing one flower in a similar container with no pumped - in carbon dioxide means this plant has a \(CO_2\) level (normal air \(CO_2\) level) that can be compared to the plants with different pumped - in \(CO_2\) levels. This is a proper control for the \(CO_2\) variable.

Answer:

She should place one flower in a similar container, but not pump carbon dioxide into it.