Question: Students on two different school campuses are comparing the growth rate of grass three weeks after fertilizer has been applied. The same fertilizer and the same amount of water are used on both campuses. Which additional variable is most important to control when the results of the two investigations are compared?

  Options: 
 A. type of grass used
 B. amount of trees in the area
 C. the weather conditions of the day
 D. the time of day the measurements are taken
Answer: A