Digital signatures and weak hash functions

There was this question in last year's exam, which I'm not totally sure I would have answered correctly (the answer b), which is supposed to be correct is bolded):

Attacks are constantly being performed on digital signatures which use weak hash functions (e.g. MD5). When an exploit of a hash algorithm is found,...:

a) all the signed documents are equally exposed.

b) the newly signed documents are more exposed.

c) the older documents are more exposed.

d) the older documents are a little more exposed.

Upon reading the question I first would have went with answer a), since I don't know what time has to do with a vulnerability in a hash function. But then I thought maybe it has something to do with the expiration date of the involved certificates (and public keys). It this why b) is, supposedly, the correct answer?

Thank you in advance.