The correct answer is C. <span>She lives on the twentieth floor of an old apartment building somewhere in Manhattan.
First of all, in front of 'twentieth floor,' there has to be the article THE, because there is only one 20th floor in a building. Additionally, in front of 'old apartment building,' there has to be the article AN, because we don't know which building exactly + the word starts with a vowel, so it has to be AN rather than A.
</span>
