In October 2001 at the site of the World Trade Center in New York City, fires were still burning from the terrorist attacks that had brought down the buildings. The greatest danger to the respiratory system was

toxic dust particles

Explanation:

After the World  Trade Centre buildings collapsed on September 11, 2001, dust particles resulting from crushed concretes mixed with other substances formed toxic were released into the atmosphere.

And these toxic particles post the greatest danger to the respiratory system of those lacking effective personal respiratory protective equipment.

it is prolly d and c and f

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.