Answer :
To increase the validity of his data, Mario could add a 0 hours control. Including a control group that receives no light provides a baseline measurement. This allows for a comparison between plants that receive varying amounts of light and those that receive none. Having a control group is essential in experiments as it helps to ensure that the observed effects (in this case, the volume of oxygen produced) are due to the variable being tested (the amount of light) and not other factors.
Reducing the number of plants would likely decrease the validity because it would provide fewer data points and could make the results less reliable. Removing the 1 hour group would reduce the range of data and may overlook the effects of minimal light exposure. Measuring the oxygen in liters rather than milliliters wouldn't necessarily increase the validity; it would just be a different unit of measurement and would not affect the accuracy or reliability of the data.
Reducing the number of plants would likely decrease the validity because it would provide fewer data points and could make the results less reliable. Removing the 1 hour group would reduce the range of data and may overlook the effects of minimal light exposure. Measuring the oxygen in liters rather than milliliters wouldn't necessarily increase the validity; it would just be a different unit of measurement and would not affect the accuracy or reliability of the data.