2.  Dashes (—) separate a word or group of words from the rest of the sentence.  Dashes are used either to indicate an abrupt break in thought or to introduce an explanation or afterthought.

3.  I tried to express my gratitude not that any words could be adequate but she just nodded and walked away.  The clause “not that any words could be adequate” must be isolated form the rest of the sentence.  I tried to express my gratitude — not that any words could be adequate - but she just nodded and walked away.

4.  When the group of words that needs isolating is in the middle of a sentence, dashes function as a pair of less formal parentheses.  When the phrase that needs isolating is at the end of the sentence instead, only one dash is required.  Just outside the door to the cabin we heard the howling of wolves — a sound that made our hair stand on end.

5.  If the underlined portion or any of the answer choices contains a dash, compare the dash to the punctuation marks available in the other answer choices.  Check the non-underlined portion of the passage for dashes that might be linking up with this one to isolate a clause or phrase.  Ask yourself whether the sentence contains a sudden break in thought, an explanation, or an afterthought.

6.  Martz, Geoff, Kim Magloire, and Theodore Silver. Cracking the ACT. 2007 ed. New York: Random House, 2007. Print.