Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances, the cost of producing electric power at photovoltaic power plants, allowing for both construction and operating costs, is one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants, which burn fossil fuels, has increased. Thus, photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.
The conclusion of the argument is properly drawn if which one of the following is assumed?
(A) The cost of producing electric power at traditional plants has increased over the past 20 years.
(B) Twenty years ago, traditional power plants were producing 10 times more electric power than were photovoltaic plants.
(C) None of the recent technological advances in producing electric power at photovoltaic plants can be applied to producing power at traditional plants.
(D) Twenty years ago, the cost of producing electric power at photovoltaic plants was less than 20 times the cost of producing power at traditional plants.
(E) The cost of producing electric power at photovoltaic plants is expected to decrease further, while the cost of producing power at traditional plants is not expected to decrease.

its asking about the asumption, so it should be C or D.

D seems better

You are right. With the Assumption stated in D. Photo plants may or may not be less expensive. In your example,
Photo is less than 2
Traditional is greater than 1.

The conclusion there is that either could be less expensive. The error in D is that it says was less than 20 times the cost . If it just said was 20 times the cost then I would be more convinced.

But I still think D is the closest, I cannot convince myself that any of the other answer choices are correct.

AK, what is the source? what is the OA?

oa is D

source is LSAT.