<p>George Thornton Emmons was one of a handful of ethnographers who committed their lives
---------------------------A------------------------------------------B----------C------</p>
<p>to studying the Tlingit culture of the Northwest Coast.
-----D---------------------------------------------</p>
<p>The correct answer is apparently C because "here is noun-noun disagreement. The singular noun "life" does not agree with the plural noun "ethnographers.""
BUT.... I though you were supposed to ignore prepositional phrases/ modifiers when trying to determine whether something is singular or plural. ex "A group of party animals goes home."</p>