Arithmetic growth of plants is mathematically expressed by the given equation

${\mathrm{L}}_{\mathrm{t}}={\mathrm{L}}_{\mathrm{o}}+\mathrm{rt}$

What does '$\mathrm{r}$' indicates in this equation.

(a) Growth in plants is internal/intrinsic and open-ended.

(b) Formation of cellular materials is called real or protoplasmic growth.

(c) Plant growth is diffused only during the early embryonic stage.