Answer:
<em><u>Although</u></em> she was tired, she read until midnight.
Explanation:
The word "although" is a subordinating conjunction that presents a dependent clause. It is used to present a statement that contrasts the main idea of the sentence, introducing the dependent clause to complete the information of the independent clause.
So, the given sentence<em> "_______ she was tired, she read until midnight"</em> presents two contrasting yet connected sentences. While <em>"____ she was tired"</em> presents the contrast in the final clause<em> "she read until midnight"</em>, using "although" will complete the whole sentence.
Thus, the final sentence will be "<u>Although</u> she was tired, she read until midnight."
