In the following reactions:

I. $\text{Ph-CH}_2\text{CH}_2\text{Br} \xrightarrow{\text{Alc. KOH}} \text{Ph-CH}=\text{CH}_2$

II. $\text{Ph-D-CH}_2\text{CH}_2\text{Br} \xrightarrow{\text{Alc. KOH}} \text{Ph-D-CH}=\text{CH}_2$

The rates of reactions (I) and (II) are the same. Both reactions proceed by which mechanism?