Than exist or than exists?

Hi, everyone.
Today my fellow teachers asked me which of the four options is the correct one for the blank in the following multiple choice question:

These proposals sought to place greater restrictions on the use and copying of digital information than ______ in traditional media.
A. exist B. exists C. existing D. to exist

I chose A and my reason is that "than exist in traditional media" in this case means "than those which exist in traditional media" or "than the restrictions we have in traditional media", but I'm not so sure of it. I would like you to help me with it.

I have come across the same kind of sentence before and have taken notes of them. I have got one taken from the Oxford dictionary, p. 284, which goes like this:
Circumlocation means using more words than are necessary, instead of speaking or writing in a clear, direct way.

There's another example sentence taken from a book on English writing by a Chinese professor of linguistics:
You can make your definitions reasonably complete by using as many techniques of defining as are necessary.

A famous Chinese professor named Chuangui Ge (???),who passed away in 1992, points out in his The Writing of English (It is an English book intended for Chinese learners of English) that "that which" might be supplied after "than"/ "as" in each sentence above.

Please tell me, how do you native speakers interpret such than-/ as- structures? Most importantly, how do you decide whether the finite verb following "than" or "as" in such sentences is in its thid person singular form or in its third person plural form?
Thanks.
Richard