This returns a list of sentences that contain a specified string.

SentenceExtractor.cs

public static List<String> SentenceExtractor(this string text, string word)
{
    var sentences = text.Split(new[] { ". " }, StringSplitOptions.RemoveEmptyEntries);
    var matches = from sentence in sentences
                  where sentence.ToLower().Contains(word.ToLower())
                  select sentence;

    return matches.ToList();

}