This was a fun question
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ANSWER: E
On my first reading I knew it was (E), but I didn't like the "preventing death" part. I went back and eliminated all the other ones, and re-read (E), making sense of the "preventing death" part. My reasoning was as follows:
Amaryllis go dormant during the dry season. It we want amaryllis to thrive as houseplants we should also keep them dormant.
There is a little disconnect here, and thus, to be valid, the argument requires a little more. So where's the gap? We want the amaryllis to thrive indoors, but nowhere in the argument does it say that an outdoor amaryllis, in order to thrive, must go dormant.
We are looking for an answer choice that plugs up this gap.
With that in mind I eliminated as follows:
(A) Most kinds of plants go dormant at some time or other during the year.Who cares about other plants (I mean question-wise, not in general)
(B) Amaryllis are more difficult keep as houseplants than other kinds of plants are.Again, who cares about other plants.
(C) Water should be withheld from amaryllis plants kept as houseplants during the exact time of year that corresponds to the dry season in their native habitat.
This may help the amaryllis go dormant (or it may not). It definitely doesn't bridge the gap, i.e., does dormancy allow an amaryllis to thrive.
(D) Any amaryllis plant that fails to thrive is likely to have been dormant for too short a time.This answer choice shows the importance between dormancy and thriving. However, it does not address the assumption: in order to thrive an amaryllis must first go dormant.
(E) Going dormant benefits amaryllis plants in their native habitat in some way other than simply preventing death during overly dry periods. ANSWER
The key here is connecting going dormant with thriving. This answer suggests that the amaryllis doesn't just go dormant in order to survive. It leaves open the possibility that the amaryllis goes dormant in order to thrive. That is, if you want an amaryllis to thrive--either outdoors or indoors--you have to make it go dormant.