I would like to understand which preposition to use with "contribution" when it is used in a non-monetary sense in American English in the case when the preposition is followed by a gerund phrase. My example is:
The work of X is an important contribution to/toward/towards a better understanding of White-Zweistein manifolds.
The pages http://www.oxfordlearnersdictionaries.com/definition/english/contribution, "Contribute for" or "Contribute to" or "Contribute on" or "Contribute in" and "Toward" or "towards"? seem to be of different opinions with varying degree of explanation. Regarding AmE, I found out that "toward" sounds better that "towards", but I could not find out the relative order between
- "contribution to ‹gerund phrase›" and "contribution toward ‹gerund phrase›"
as well as the relative order between
- "contribution to ‹gerund phrase›" and "contribution towards ‹gerund phrase›"
in the non-monetary sense.
Which preposition would sound best in the above example and why?