Sentence Correction (SC) | Re: The massive Himalayan avalanche surprised climate scientists, who saw

The massive Himalayan avalanche surprised climate scientists, whosaw a collapse that big and that fast – over 250 million cubic feet of snow and ice moving at speeds over 200 miles an hour – only once before.

A. saw a collapse that big and that fast --> wrong: see the explanation
B.having seen a collapse that big and fast --> no verb
C.were seeing a collapse that big and fast --> unnecessary past continuous tense
D.will see a collapse that big and that fast --> happened in the
...

---