求一道语法题

sissi8466

Scientists have recentlydiscovered what could be the largest and oldest living organism on Earth, agiant fungus that is an interwoven filigree of mushrooms and rootlike tentaclesspawned by a single fertilized spore some 10,000 years ago and extendingfor more than 30 acres in the soil of a Michigan forest.  
A. extending
B. extends
C. extended
D. it extended
E. is extending
我选的C, 但答案选A. 有点不明白为什么用ing形式，请指教下。

jj449

1.spawned 表被动；extending 表主动，都是修饰interwoven filigree的
2.用extending表明从过去的一个小孢子，扩展到今天有30英亩那么大。它没灭绝，因此是从过去到现在一直进行的动作。