Only when I read the Frog by Moyan a second time _________ to appreciate his magic realism.

A．did I begin

B．I began

C．had I begun

D．I have begun

集成运放的非线性应用是指运放工作在其特性的非线性区，运放内部的三极管都工作在（），运放的输入输出为非线性关系。