Re:OG SC 210、どなたか分かりますか？

> 210. Scientists have recently discovered what could be the largest and oldest living organism on Earth, a giant fungus that is an interwoven filigree of mushrooms and rootlike tentacles spawned by a single fertilizer spore some 10,000 years ago and <extending> for more than 30 acres in the soil of a Michigan forest.
> A) extending
> B) extends
> C)it extended
> D)it extended
> E)is extending
>
> 正解はAで分かるんですが、Eの駄目な理由が分かりません。どなたか教えてもらえますか。

このextendingはspawnedと並列で、名詞rootを後ろから修飾する語法です。
故にbe動詞は不要です。