Which of the following substances will produce acetaldehyde in the presence of an aqueous solution?

In the reaction sequence:
$\text{Ethanol}$ $\xrightarrow{PCl_5} X$ $\xrightarrow{\text{alc. } KOH} Y$ $\xrightarrow[H_2O, \Delta]{H_2SO_4, \text{room temp.}} Z$
The product $Z$ is:

Explain the acidic dehydration of alcohols with an example.

The products obtained in the following reaction are:

$HO-CH_2-CH_2-OH$ on heating with periodic acid gives:

Which of the following alcohols undergoes dehydration most easily with concentrated $H_2SO_4$?