If you mean an adverb clause, I'd say your answer is D. Adverb clauses typically begin with a subordinating conjunction like because, after, if, or although.
Every car, truck and bus go across this bridge to reach the island.
OH YEAH DO YOU SEE THIS DO YOU SEE THIS OPEN YOUR EYES SHIKAMARU ITS A **REGULATION HEADBAND
Answer:
They slow down when travelling through material such as air.
Explanation: