Section: Critical Reading Section 2

104) The referee halted play in an effort at avoiding serious altercations in the stadium between fans of the opposing teams.

1. at avoiding serious altercations in the stadium between fans of
2. at avoiding a serious altercation in the stadium between fans of
3. to avoid serious altercations in the stadium between fans of
4. to avoid serious altercations in the stadium between fans on
5. to avoid serious altercations between fans in the stadium of

In this sentence we must sort through all the prepositions to see that in an effort at avoiding is not good, idiomatic English. The correct phrase is in an effort to avoid.