Why is it sometimes difficult for ecologists to design controlled experiments?

Which substance contains thylakoids? a) nadph b)atp c)stroma d)chloroplast

Where is most of the fresh water on earth found a in the ocean b in glaciers and in ice caps c in rivers d in the soil

Asap many people try to eliminate fat from their diets which is one reason it is necessary for humans to eat fat a) fat is the only way to get energy b) every cell in the body must contain fat c)fat stores less energy than polysaccharides d) saturated fats make blood vessels healthier