An interior point cannot be an extreme point. 一個內點不可能是極點。
The vital threshold is crossed when the plant population shrinks to the point that the recycled rainfall becomes too meager to sustain the population. 當植物的群落萎縮到了極點，以致循環降雨過少，難以維繫群落的生存時，維持生命的門檻就被跨越了。
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