答案it 指代the severe snow storm;
it caused to the...是省略了关系代词的定从, 因为the sufferings在定从中作宾语.即:
the severe snow storm caused the sufferings to the... 凶猛的暴风雪给(人们)造成了灾难.
如变成that, 也是定从, that指代the sufferings, 在定从中作主语.
那就成了: the sufferings caused the sufferings to the... 这显然不通.
因此: 答案为it .