Final answer:
The number of new brain cells per square centimeter of cortical space in Dr. Farrell's study is a variable.
Step-by-step explanation:
The number of new brain cells per square centimeter of cortical space in Dr. Farrell's study would be considered a variable. A variable is any factor that can change or be changed in an experiment. In this case, the variable would be the number of new brain cells, which is being measured as a result of the daily dose of citrus fruit intake. By studying the effects of citrus fruit on neurogenesis, Dr. Farrell is trying to determine the impact of this variable on brain cell growth.