Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances...

SamA on September 24, 2019

Hello @phillip-blackmar,

The premise only tells us how the prices of photovoltaic power plants and traditional plants have each changed over the years. It does not tell us who the prices compare to one another.

Premise: Photovoltaic is now 1/10 of the price that it was 20 years ago.

Premise: The cost of traditional plants has increased in the last twenty years.

Conclusion: Photovoltaic plants are now less expensive.

In order to draw this conclusion, we need information that compares the prices of the two types. An assumption that demonstrates that photovoltaic power is currently cheaper is what we should look for among the answer choices.

Answer choice D gives us what we are looking for, because it provides a link between these two price points. If photovoltaic plants used to be less than 10x the cost of traditional plants, then it would now be cheaper since it is 1/10 of its old cost.

If this were not true, and photovoltaic used to be 11x more expensive than traditional, this argument would fall apart. Remember that the correct choice in assumption questions will always be essential to the validity of the conclusion.