Over the past 20 years, the major pollutants in our world have
a. decreased significantly
b. increased significantly
c. stayed at the same levels
d. decreased slightly

The most likely answer is B, increased significantly. This is so, because of the vast amount of cars, and factories, still in use.So, based upon the statistics and the levels of pollution all over the world, I would have to answer B. 

If the experiment was perfomed more then once and you get the same results

What orders do you have in your chart