Technological improvements and reduced equipment costs have made converting solar energy directly into
electricity far more cost-efficient in the last decade. However, the threshold of economic viability for solar power
(that is, the price per barrel to which oil would have to rise in order for new solar power plants to be more
economical than new oil-fired power plants) is unchanged at thirty-five dollars.
Which of the following, if true, does most to help explain why the increased cost-efficiency of solar power has
not decreased its threshold of economic viability?
(A) The cost of oil has fallen dramatically.
(B) The reduction in the cost of solar-power equipment has occurred despite increased raw material costs
for that equipment.
(C) Technological changes have increased the efficiency of oil-fired power plants.
(D) Most electricity is generated by coal-fired or nuclear, rather than oil-fired, power plants.
(E) When the price of oil increases, reserves of oil not previously worth exploiting become economically
viable.

From what i understand economic viability in this case means monetery advantage of solar power plant over that of oil-fired plants has been constant in spite of the fact that technology has made it cheoper to convert solar energy to electricity. I don't understand what the following sentence means
(that is, the price per barrel to which oil would have to rise in order for new solar power plants to be more
economical than new oil-fired power plants)
Could you please expain to me