Answer: Construction of bridges : Water pollution
Explanation: Bridges are built over water, correct? So you can only assume that debris is going into the water when being constructed.
Also, you can terminate the other answers immediately
1. Decrease in honeybee population : Increase in plant population
That would decrease the plant population because honeybees are needed in order for plants to survive.
2. Use of fertilizers : Increase in population of aquatic animals
This would decrease the population of aquatic animals because fertilizers are toxic, and runoff from farms can go into water and harm aquatic animals.
3. Construction of dams : Decrease in carbon dioxide level
Construction of dams don't decrease carbon dioxide levels. If anything, it'd cause pollution.
