When we want to EVALUATE a conclusion, we need to look for the LOGICAL GAP between the premises and the conclusion.

Premises:
- emission of sulfur dioxide from burning high-sulfur coal is restricted
- new plants usually install expensive equipment to filter sulfur dioxide form emissions

Conclusion:
Plants could save money by installing less expensive equipment that removes sulfur before combustion

Logical Gap:
What about other factors that affect cost? We're given info about comparative equipment costs, but those aren't the only costs involved. What if the process or the labor costs more? Or if we'd need to use more coal? To evaluate whether they'd save money, we need information about other costs.

A. Oil-burning plants are irrelevant to the cost of coal-burning plants
B. The comparison of new plants v. old plants is a false comparison (a common CR wrong answer type). The conclusion specifically claims that "new plants could save money," so we don't care about the old ones.
C. This speaks to another cost - the cost of the process! This speaks to the logical gap that we were missing, so it is useful to evaluate. CORRECT
D. In the context of the argument, we don't care about what the emissions are doing to the environment (although hopefully we care in real life!). It's not relevant to this conclusion, so it's out.
E. Again, our conclusion specifically addresses NEW plants, so we don't care what existing plants do, or whether they can compete.