How error function can be calculated in diffusion phenomena?

A. $$erf\left( {\frac{x}{{2\sqrt {Dt} }}} \right) = \frac{2}{{\sqrt \pi }}\int\limits_0^{\frac{X}{{2\sqrt {Dt} }}} {\exp \left( { - {\eta ^2}} \right)d\eta } $$

B. $$erf\left( x \right) = \frac{2}{{\sqrt \pi }}\int\limits_0^{\frac{X}{{2\sqrt {Dt} }}} {\exp \left( { - {\eta ^2}} \right)d\eta } $$

C. $$erf\left( t \right) = \frac{2}{{\sqrt \pi }}\int\limits_0^{\frac{X}{{2\sqrt {Dt} }}} {\exp \left( d \right)d\eta } $$

D. None of these

Answer: Option A