如果对它们的数量进行时间记录，就会发现它是很不稳定的——以不规则的周期急剧上升下降。
If their population is tracked through time, it will be seen to be particularly unstable—soaring and plummeting in irregular cycles.
许多州的寄养人口目前依然稳定或者正在上升。
Many states still are experiencing stable or rising foster care populations.
识字率上升；预期寿命上升；家庭规模稳定。
Literacy is up; life expectancy is up;family size is stabilising.
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