ANSWER ASAP IM GIVING BRAINLIEST Which of the following sentences has a subordinating conjunction that introduces a dependent clause?

A.
Before advanced computers existed, Arthur dreamed of making space travel a reality.

B.
In his free time, Arthur worked in the areas of study he loved.

C.
Arthur studied and used his scientific knowledge throughout his entire life.

D.
Arthur built on technology systems that already existed to create his new system.