Which of the following is a complex sentence?
A. I brushed my teeth while the coffee brewed.
B. Time was running short, so I hurriedly brushed my teeth while breakfast was warming up.
C. Since time was running short, I hurriedly brushed my teeth while breakfast was warming up.
D. I used the time while the coffee was brewing to brush my teeth.
A complex sentence is one that contains an independent clause and at least one dependent clause.
In Choice C: "Since time was running short, I hurriedly brushed my teeth while breakfast was warming up," we have an independent clause: "I hurriedly brushed my teeth," and a dependent clause: "Since time was running short." The dependent clause cannot stand alone as a complete sentence, so it depends on the independent clause for its meaning. This combination of an independent and dependent clause makes Choice C a complex sentence.
The other choices are not complex sentences because they either lack a dependent clause or do not have the same level of complexity in their structure.
Therefore, the Correct Answer is C.