Which of the following alcohols is dehydrated most easily by warm dilute acid-catalysed E1 dehydration (same acid, same temperature)?

Acid-catalysed dehydration proceeds through the carbocation, so the ease follows carbocation stability: tertiary $>$ secondary $>$ primary. 2-Methylpropan-2-ol gives the tertiary cation and dehydrates at the lowest temperature (about 360 K with 20% $\mathrm{H}{}_2\mathrm{SO}{}_4$), while ethanol needs 440 K with concentrated acid.