Task: Question Answering
Input (English): Background Paragraph: Collision theory tells us that molecules need to collide in order for a reaction to take place. All else being equal, anything that increases the frequency of collisions would also increase the rate of reaction. One way to do this is to increase the concentrations of one or more of the reactants. Using a real world example, a busy freeway will experience much more frequent car collisions than a fairly empty neighborhood street. Similarly, forcing more randomly moving molecules into the same amount of space will cause an increased number of collisions over a given length of time, providing the reactants with more opportunities to transform into products. 
Story: Rob wanted to learn more about collision theory. For that, he conducted two tests, test A and test B. In test A he used lower concentration of reactants. In test B he used higher concentration of reactants. Now, he needs to know about the science behind these tests. 
Question: Which test would decrease the frequency of collisions among molecules, test A or test B?
Output (English): test A.