"The plan has good chances of success, since sweet potato is a staple of the region's diet and agriculture, and the varieties currently grown contain little beta-carotene"
Choice D: There are no other varieties that are significantly richer in beta-carotene than SPK004 is.
Choice D is just a re-phrase of the bolded item. The question is to strengthen the argument. So, the choice should not steal the same idea from the stimulus. The choice must include extra information, yet it must be tangential to the stimulus. So, E is the best choice.
A,B and C are totally absurd and can be eliminated straight away.
