Various colors of light are tested to see which growth promotes the best a. Name the independent variable.
b. Name the dependent variable.

You need more context to the question, however I think I have it figured out.
Independent variable- various colors of light
dependent variable- The growth rate from each color

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.

Carbon monoxide is the answer: )