[单选题]
A transformer is a dev(.d/mez yzscpb/m z+ 7k3nn y.ice used t6l,xck3j g*)3sfknw xpr :1wno
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.
