placebos are used in experiments to group of answer choices control for the variability of individuals who tend to volunteer for research studies. control for the expectations of some research participants that they will improve just because they are in a research study. determine whether an independent variable actually causes a statistically significant change in a dependent variable. make certain that the treatment and control group are randomly selected.

b is the correct answer

answer;

c. a tree whose roots are surrounded by wet sand.

explanation;

water stress is an important factor limiting the growth and productivity of forests.water stress is the most often cited cause of poor survival and growth of woody landscape plants.

-insufficient soil moisture is the most common form of water stress, but excess moisture that leads to poor soil aeration (hypoxia) can also result in reduced plant vitality. a tree whose roots are surrounded by wet sand would be least likely to experience water stress.