88. A transformer is a passive electrical device that transfers electrical energy from oneelectrical circuit to another. A varying current in any one coil of the transformerproduces a varying magnetic flux in the transformer's core, which induces a varyingelectromotive force across any other coils wound around the same core. Transformersare commonly used in our daily life, and provide the voltage we need, for example, from220 volt to 110 volt. Such emerging work is achieved by varying                 .
(A)the size of transformer's magnetic core
(B)the number of turns in a winding
(C)the magnetic flux in the core
(D)the material of the core