Rule 13. Sometimes the pronoun who, that, or which is the subject of a verb in the middle of the sentence. The pronouns who, that, and which become singular or plural according to the noun directly in front of them. So, if that noun is singular, use a singular verb. If it is plural, use a plural verb.
Examples: Salma is the scientist who writes/write the reports.
The word in front of who is scientist, which is singular. Therefore, use the singular verb writes.
He is one of the men who does/do the work.
The word in front of who is men, which is plural. Therefore, use the plural verb do.
Now for an SAT question:
George Thornton Emmons was one of a handful of ethnographers who committedtheir lifeto studying the Tlingit culture of the Northwest coast
This was identifying sentence errors, now say the "who" wasn't there and this was in the present, commit would be in the plural still right? If this were true why isn't "life" also plural? Or is this only a subject-verb rule and the antecedents still refer back to the George who is only one person?