Answer: Transformation of one thing to another follow cyclic pathway.
Explanation:
In biology the arrows are used to show the conservation of one form of energy into another form. This can be seen in the cyclic pathways in which transformation of one thing occurs into another thing. For example the concept of metamorphosis in insects involves the conversion of transitional immature stage into adult insect by egg, pupa, larva then into adult insect this growth can be represented by the series of arrows.
First, he should measure how long it takes for the liquid to become clear if X and Y are mixed together. Then, he should measure how long it takes if he also adds substance A to X and Y. He will find out if substance A is a catalyst.