Final answer:
The correct placement of the dash in the sentence 'Oh, my! Look what the cat dragged in a seventy-pound rat!' is after 'Oh, my!', resulting in 'Oh, my!—Look what the cat dragged in a seventy-pound rat!'.
Step-by-step explanation:
The question pertains to the correct placement of dashes within a sentence for emphasis or clarification. When deciding where to place a dash in the provided sentence 'Oh, my! Look what the cat dragged in a seventy-pound rat!', we need to consider the phrase that is being emphasized or set off from the rest of the sentence.
The correct placement of the dash(es) in the sentence is after 'Oh, my!', turning the sentence into 'Oh, my!—Look what the cat dragged in a seventy-pound rat!'. The dash after 'Oh, my!' serves to provide emphasis and to set off the interjection from the surprising or amusing statement that follows.