bład w zdaniu ??

w ksiazce grammar and Vocabulary for cambridge advanced and proficiency bylo zdanie
"It is primarily its greater size WHICH distinguishes the raven from the common crow"
dlaczegfo nie ma tu cleft sentence czyli It is primarily its greater size THAT.....?? przeciez gdy mamy It jako introductory poznierj jest that!

Frankly, the sentence sounds correct to me. I'd naturally use 'which' instead of 'that'. Maybe it's just a matter of British vs American English?

Seems I was wrong. A native said it reads awkwardly with which and 'that' is the way to go. :)

Well, I'm not an expert, but as far as I know, both forms are correct. You may find such sentences with "which" even in good literature, like in a translation of Euripides:
"It is Love which causes Earth to yearn for rain, when the parched ground, barren with drought, has need of moisture. It is Love which makes the sacred Heaven, swollen with rain, to sink into the lap of Earth."

Anyway, I'd like to hear from someone knowing the subject matter better than myself too check if this is true:)

I've done some research into it and it appears that which is more formal, but 'that' reads more naturally, the former is considered quite dated.

... but AS 'that' reads more naturally ...