光速为什么用C表示？

Why is c the symbol for the speed of light?
The symbol c seems to have been used for the speed of light (in vacuum) for a long time. Einstein used other symbols such as V in his early relativity papers but most early books on relativity used c.
It probably stands for the Latin word celeritas meaning swift. Celerity is also an infrequently used word in English for speed and the same root is usedin the words accelerate and decelerate.
It would be interesting to know more about who used c for the speed of light(or other speeds) before the twentieth century.

当用D的时候你又会问:光速为什么用D表示!!!!!!!!!!!!

家用电器用的插座与电灯之间是并联
这样连接使插座与电灯相互不影响，如果串联的话，插座不通电，电灯就不亮，通了电，电灯因通过的电流小也有可能很暗。
反之，电灯的开关不打开，插座也无法使用。
可见，这样连接可以防止电灯与插座之间相互影响。

源自拉丁文celeritas