Question: No less impressive were the secular challenges.
The secular challenges were not as impressive.

Answer: no

Question: All of that was then.
All of that happened back then.

Answer: yes

Question: As indicated in the table, the Clear Skies Act yields relatively modest air quality improvements for about one-fourth of the US population (i.e.
The Clear Skies Act doesn't enforce strong enough measures.

Answer:
it is not possible to tell