Question

Dear Redwood Girl,

Good morning!
Furthermore, on active/passive-
Wren & Martin Ex 41-8 change to passive-
"They saw the storm approaching."
The key says-
"The storm was seen approaching." (seems perfect to me.)

But when you converted-
"They saw the clouds thundering." became,
"The thundering clouds were seen" instead of,
"The clouds were seen thundering."
("thundering" became adjective.)

Is it because "thundering" could not be seen but heard only? or the question itself was structured wrongly? (should have been "they saw the thundering clouds")

Thanks...