We　are　used　to　the　idea　of　aging　in　ourselves.　We　are　so　used　to　this　that　it　comes　as　a　surprise　to　find　that　there　may　be　some　animals　that　do　not　age.　Sea　anemones(海葵)　are　an　example.　Some　have　been　kept　for　nearly　a　century　without　showing　any　signs　of　lifelessness.　Some　kinds　of　sea　worms　can　even　“grow　backwards.”　If　kept　in　the　dark　and　given　nothing　to　eat,　they　get　steadily　smaller,　They　finally　end　as　a　ball　of　cells(细胞)　looking　rather　like　the　egg　from　which　they　came.　Under　good　conditions　the　ball　will　turn　back　to　a　worm　and　start　growing　again.　One　could　probably　keep　them　growing　and　un-growing　again　and　again.The　underlined　word　aging　in　the　first　sentence　means　______.　　　　

A.
growing old


B.
the age of a person

C.
getting younger

D.
un -growing

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