高压侧为单电源，低压侧无电源的降压变压器，( )装设专门的零序保护。
A．不应 B．可 C．宜 D．不宜
参考答案：D
现代金融学关于证券估值的讨论，基本运用各主观假设变量，结合相关金融原理或估值模型，得出"理论价格"作为证券的"内在价值"。（）
. I first met Mr Zhao nine years ago. He ____ at a chemical college at that time.
A．had worked
B．has worked
C．was working
D．had been working
