Answer: The various variables can be grouped as :
Independent variable; Humidity levels
Dependent variable; Heights of the trees
Controlled variable ; Green house
Explanation: The independent variable is the variable that not affected by the change in other variables, it determines the outcome of an independent variable in an experiment. and from our question the independent variable is the different humidity levels inside the Greenhouse
Dependent variable is the measured variable in an experiment which is primarily dependent on the Independent variable, it can be likened to be effect/result of an experiment ; height of the trees
A controlled variable is a variable that is held constant during an experiment because it doesn't have a direct importance on the aim of the experiment, but it can affect the outcome if not controlled ; Green house
Hence the independent variable = Humidity levels, Dependent variable = height of trees and the controlled variable = green house