Which of the following is a complex sentence?
A. I brushed my teeth while the coffee brewed because time was running short
B. Time was running short, so I hurriedly brushed my teeth while breakfast was warming up.
C. I used the time while the coffee was brewing to brush my teeth
D. I hurriedly brushed my teeth while breakfast was warming up since time was running short.
Is complex as it contains an independent clause ("I hurriedly brushed my teeth while breakfast was warming up") and a dependent clause ("Since time was running short") linked by the subordinating conjunction "since." This combination makes it a complex sentence.
Therefore, the Correct Answer is D.