It is possible to grow agricultural crops that can thrive when irrigated with seawater.
Conclusion
Farming in which crops are irrigated with seawater, if done near oceans, would actually be cheaper than most other irrigated agriculture.
Evidence (since)
The greatest expense in irrigated agriculture is in pumping water, the pumping costs increase with the distance water is pumped, and the water wouldn't have to be pumped far (if done near oceans).
Evaluate
Whenever we read the argument on Role or Main Conclusion, we prime our brain with the expectation that first we'll see the Main Conclusion and then we'll hear the supporting ideas ("upside-down arguments").
In this argument, the first sentence sounds like it might be an opinion, but the author doesn't try to convince us that it's possible to grow crops irrigated with seawater.
Meanwhile, the 2nd claim, that such farming would be cheaper is immediately followed by the Support Indicator 'since' (FABS = for, after all, because, since), which shows us that this 2nd claim is probably the conclusion.
Our theory is confirmed when we see that the final sentence also is supporting that idea that irrigating with ocean water, if done near the ocean, would be cheaper.
The last sentence has two claims, joined by "and". The word 'and' (99% of the time) joins together two supporting claims.
Goal
We're being asked about the 2nd to last claim, which is one of our 3 premises.
So look for an answer like, .