潜影是由下列哪个组成的
A．感光中心B．显影中心C．银离子D．溴离子E．电子
参考答案：B
解析：许多的显影中心构成了潜影。
_____ homework, they do their homework on Sunday. [ ]
A. As for
B. As to
C. As of
D. As from
堵塞
