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

Oh, they definitely have increased significantly!

last 20 years means since 1995; and until recently people couldn't even agree whether there is man-made global warming, so there wasn't even a consensus whether we have to decrease the pollution!

Right now, we are talking of decreasing the rate of increase of pollution, not even of decreasing the pollution itself!

It might seem that the developed countries are decreasing their pollution, but this is not the case in the developing countries which increase their pollution as they develop.