简述照相机的三个发展阶段。
参考答案：第一阶段（1839-1954）
第二阶段（1954-1975）
第三阶段（1975-今）
—Hi, Jim. What about the weather in London?
—Terrible! ______. So I have to stay at home. [ ]
A. It's rainy
B. It's sunny
C. It's cloudy
D. It's fine
简述拥塞管理技术。
