[单选题]
A transformer is a da,viogy.bwhd *ec /l7* ac(d)evice used to ,+k-)wh3 w/+yk a/bfasevh ij
A. transform an alternating current into a direct current.
B. transform a direct current into an alternating current.
C. increase or decrease an ac voltage.
D. increase or decrease a dc voltage.
