Councilperson X: We have an obligation to help ensure that electricity rates are the lowest possible.
X's 1st Idea
We're obliged to ensure that electricity rates are the lowest possible. Does Y disagree?
No, Y seems to agree. She is saying "that's not our only job", but this implies that "yes, this is one of our jobs".
X's 2nd Idea
The proposed design for a new generating station would allow for the lowest rates. Does Y disagree?
No, Y seems to agree. She acknowledges that such a design would have "the benefit of lower rates".
X's 3rd Idea
This proposed design must be the design we endorse, if we agree that we're making a new plant. Does Y disagree?
Yes, Y seems to agree. She is saying, assuming we had decided to approve construction of a new plant, we do not necessarily have to or want to pick that design.
Even though we're obliged to ensure low rates, we're also obliged not to lower the quality of life in our community, and this new design would damage air quality.
Evaluate / Goal
This proposed design pits two competing obligations against each other, says Y. The design's ability to satisfy one goal is outweighed by its potential to violate our other goal.
In X's mind, since the proposed design would best achieve our obliged goal of lowest rates, it's our obliged choice if we're choosing a new plant.
We should keep in mind that no one ever said they are approving construction of a new plant. This disagreement is happening in a hypothetical world, where we are deciding on a design for a new plant.
X would say, in that case, "we must choose this new proposed design" and Y would say "no way."