20. 直流無刷電動機的換向開關不可以使用下列何者元件？(A)雙極性接面電晶體(Bipolar Junction Transistor, BJT)；(B)金屬氧化物半導體場效電晶體(Metal-Oxide-Semiconductor FieldEffect Transistor, MOSFET)；(C)絕緣柵雙極電晶體(Insulated Gate Bipolar Transistor, IGBT)；(D)繼電器(Relay)