Found a very nice explanation on manhattangmat.
here is the link for that
http://www.manhattangmat.com/forums/mos ... t3508.htmlwhoa, no, guys. that's not the issue here.
the problem is this:
the argument says
until the cost of extracting uranium from seawater can somehow be reduced, this method of obtaining uranium is unlikely to be commercially viable.
in other words, the argument is saying that the ONLY WAY that seawater extraction will be commercially viable is if the cost of that extraction comes down.
to weaken the argument, then, you need to find ANOTHER way in which seawater extraction will be commercially viable, WITHOUT a cost reduction.
this is what choice (a) does. if the uranium on land is being depleted, then extracting uranium from seawater will become "commercially viable", out of pure necessity. the cost will go up, but that's the breaks.
choice (c) doesn't actually affect the argument at all, because the argument already acknowledges that uranium will become commercially viable IF the price comes down.
--
analogy:
if i tell you this:
unless you eat my fish 'n' chips, you'll never know the true meaning of gastronomic ecstasy
then:
* if you say "eating your fish 'n' chips will give me gastronomic ecstasy", then that doesn't affect my statement at all, because you're just echoing what i've already told you.
however:
* if you say "au contraire, i can also get gastronomic ecstasy from a nice fat slice of meat lover's pizza", then that weakens my argument, because i'm telling you there's only one source of gastronomic ecstasy but you're countering with another.
substitute
fish 'n' chips --> cost increase
pizza --> depletion of land resources
gastonomic ecstasy --> commercially viable
...and there you have it.