在電晶體電壓調節器, 電壓輸出控制是由以下何者完成?In a transistor voltage regulator, the voltage output is controlled by

A、齊納二極管和電晶體.
zeners and transistors.
B、變壓器和電晶體. transformers and transistors.
C、二極管和變壓器 diodes and transformers.