烟酸缺乏会出现癞皮病，典型症状为“三D”症，即皮炎、腹泻和（）。
参考答案：痴呆
Two hundred miles ______ a long distance ______ in two hours even by a fast car.
A．are, to cover
B．are, to be covered
C．is, to cover
D．is, to be covered
真空断路器具有（）的优点。
A.体积小
B.噪音低
C.触头开距小
D.开断容量大
