Why does the symbol c usually represent speed of light in vacuum? Choose the best possible option.
(A) Einstein used the symbol c first time in his equation $\displaystyle E=m{{c}^{2}}$
(B) Einstein used the symbol c first time in his equation $\displaystyle m=\frac{{{m}_{0}}}{\sqrt{1-\frac{{{v}^{2}}}{{{c}^{2}}}}}$
(C) c is the initial letter of the word celeritas, the Latin word meaning speed
(D) c is the initial letter of the word calculus