He is very happy _______ his friends. [ ]
A. seeing
B. saw
C. to seeing
D. to see
答案:D
从波浪理论自身的构造看,它有许多不足之处。()
锗二极管的压降比硅二极管的要小,理论上锗二极管是0.3V,硅二极管是()。
A、0.4V
B、0.5V
C、0.6V
D、0.7V