In a Cox-Ross-Rubinstein Binomial Tree, the Formula for the Proportional $u = e ^ { r \Delta t }$

In a Cox-Ross-Rubinstein binomial tree, the formula for the proportional up-movement, u, is: choose one)

A) $u = e ^ { r \Delta t }$
B) $\mathcal { U } = e ^ { r - \sqrt { \Delta t } }$
C) $u = e ^ { \sigma \Delta t }$
D) $\mathcal { U } = e ^ { \sigma \sqrt { \Delta t } }$