Which among the following is the method for the preparation of ethers?

The decreasing order of reactivity of following alcohols with halo acid is

(I) $\mathrm{CH}_3 \mathrm{OH}$

(II) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OH}$

(III)

(IV)

Identify the product ' $Z$ ' in the following series of reactions.

Ethanol $\xrightarrow[\Delta]{\mathrm{SOCl}_2} \mathrm{X} \xrightarrow[\text { Dryether }]{\mathrm{Mg}} \mathrm{Y} \xrightarrow{\mathrm{NH}_3} \mathrm{Z}$

Crotonyl alcohol is an example of