GDW1-Q8

The key is A, while what I choosed was D. Could anyone help me understand why A is the best here?

Thanks a lot!

Q8:

Sulfur dioxide, a major contributor to acid
rain, is an especially serious pollutant because it diminishes the
respiratory system’s ability to deal with all other pollutants.

1. an especially serious pollutant because it diminishes the
        respiratory system’s ability to deal
2. an especially serious pollutant because of diminishing the
        respiratory system’s capability of dealing
3. an especially serious pollutant because it diminishes the
        capability of  the respiratory
        system in dealing
4. a specially serious pollutant because it diminishes the
        capability of  the respiratory
        system to deal
5. a specially serious pollutant because of diminishing the
        respiratory system’s ability to deal