Could somebody explains to me how the answer is correct?

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 fertilized 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. extended
D. it extended
E. is extending

[spoiler]OA: A[/spoiler]