Self-study
Photovoltaic power plants produce electricity from sunlight. ██ █ ██████ ██ ███████████ ██████ █████████████ █████████ ███ ████ ██ █████████ ████████ █████ ██ ████████████ █████ ███████ ████████ ███ ████ ████████████ ███ █████████ ██████ ██ █████████ ██ ████ ██ ███ ██ █████ ████ ███████ ███ █████████████ ████ ███ ███████████ ███████ █████ ████ ██████ ██████ ███ ██████████ █████ ████████████ █████ ██████ █████ █ ████ █████████ ████████ ██ ███████ ██████ ███ ███████████ ████ ██ ███████████ █████ ███████
Argument Summary
The argument concludes that generating power at photovoltaic power plants is now cheaper than at traditional power plants. How come? Because over the last 20 years, the cost of PV power has fallen by 90%. At the same time, the cost of traditional power has increased somewhat.
Objective: Identify a Sufficient Assumption
The issue is, the argument only tells us about the change in cost of these two generating methods. But if we don't know the starting costs of each one, we can't compare their current costs. For instance, if PV power started out 100 times more expensive than traditional plants, even a 90% decrease in cost could easily still leave PV power several times more expensive.
The sufficient assumption we need to find is that PV power started out no more than 10 times more expensive than traditional power. If that's the case, then the 90% decrease in cost for PV power would have been able to close the gap and leave it cheaper than traditional power, as the argument claims. Any higher, and PV power could still be more expensive, despite the cost decrease.
12.
The conclusion of the argument ██ ████████ █████ ██ █████ ███ ██ ███ █████████ ██ ████████
a
The cost of █████████ ████████ █████ ██ ███████████ ██████ ███ █████████ ████ ███ ████ ██ ██████
(A) merely restates a
b
Twenty years ago, ███████████ █████ ██████ ████ █████████ ██ █████ ████ ████████ █████ ████ ████ ████████████ ███████
The argument is only concerned with the cost of producing power, not the amount that different plants produce. How much power each plant type produced, at any time, doesn't help us establish that PV power now truly costs less than traditional power.
c
None of the ██████ █████████████ ████████ ██ █████████ ████████ █████ ██ ████████████ ██████ ███ ██ ███████ ██ █████████ █████ ██ ███████████ ███████
The argument tells us how power costs have changed for both
What we're missing in the argument is a starting point to let us compare the changes in power costs over the last 20 years. (C) doesn't give us that; at best, (C) explains why traditional power costs haven't fallen. But since that's already stated as a fact, it doesn't need further explanation.
d
Twenty years ago, ███ ████ ██ █████████ ████████ █████ ██ ████████████ ██████ ███ ████ ████ ██ █████ ███ ████ ██ █████████ █████ ██ ███████████ ███████
Let's say the cost of traditional power 20 years ago was $10 per kWh. (D) says that PV power cost at most $99.99 per kWh (less than 10 times traditional power). Fast forward 20 years: PV power now costs a tenth of what it did, i.e. $9.99 per kWh. Traditional power increased—but even if it was just by $0.01, it now stands at $10.01 compared to PV power at $9.99. This guarantees that PV power now actually costs less.
(D) is correct because the stimulus only tells us about changes in cost over time. But without a starting point, we can't compare the results of those changes. (D) fills in the starting point, and in doing so guarantees that the decrease in cost for PV power was enough to make it cheaper than traditional power.
e
The cost of █████████ ████████ █████ ██ ████████████ ██████ ██ ████████ ██ ████████ ████████ █████ ███ ████ ██ █████████ █████ ██ ███████████ ██████ ██ ███ ████████ ██ █████████
The stimulus isn't concerned with the future; it only cares which type of power costs less in the present. No matter what the future trends in power cost, that doesn't help us guarantee that PV power costs less right now.