Answer:
D. All of these
Explanation:
Mark should control (or keep constant) the location of the plants, the amount of water the plants receive daily, and the amount of fertilizer the plants recieve daily. The only variable that should change is the type of fertilizer that is given to each plant.
During a scientific experiment, only variable should be changed by the investigator. All other variables should be kept constant.
The smallest unit on the image that can be controlled.
I think its the Russian Empire, Poltava, Perislav
