geet_ge wrote:Q) Sulphur di oxide, 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.

This a is correct sentence.

My ques - why is the following wrong:
a specially serious pollutant because it diminishes the capability of the respiratory system to deal

Is it because of ability v/s capability?
an especially v/s a specially (is there any rule regarding this?)

Hi geet_ge!

I would say that the bigger issue here is the especially/specially distinction. While the roots "especial" and "special" are nearly interchangeable, the adjective forms "especially" vs. "specially" are not. In fact, the preference is to "especially". While "specially" is used more frequently in speech, it is not considered formally correct.

As for ability vs. capability, both are idiomatically correct in their forms (ability to and capability of), but the capability of phrase is wordier (so all else equal would be less preferred).

Hope this helps!

Whit