If + were is used to express contrary-to-fact in the present:
If I were a rich man, I would buy a Mercedes. (I am not a rich man.)
If + were to is used to express an unlikely condition in the future, something that could happen but probably won't:
If I were to win the lottery, I would buy a Mercedes. (It is unlikely that I will win the lottery, but it's possible.)
If + was is very rare, used only to express a likely condition in the past, something that may well have happened, but whether it did happen is unknown to the speaker:
If John was at the party last night, he probably saw my sister. (John may well have been at the party last night, but I don't know for sure.)
The ice of Greenland and Antarctica have certainly not yet melted -- we know that -- and so was is wrong. The sentence aims to say that in the unlikely event that the ice melts..., and so were to is correct. The answer is B.



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