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

a. decreased significantly

I took the test and it was correct

Explanation:

Since then emissions have been on a downward trend in these regions. At the beginning of the 21st century emissions in Northern America are at a lower level than at any time in the 20th century.

from the article: Air Pollution Our World in Data

the species living around the vents are especially good at dispersal.  

deep-sea vent originates on the floor of the ocean when seawater circulates through hot volcanic rocks, usually situated where the new oceanic crust is being produced.  

biological dispersal refers to the movement of species from their location of birth to their site of breeding, or from breeding site to another. similarly, the oceanic dispersal refers to the movement of the species from one land mass to another by sea crossing.