Which of the following halogenated compounds is not used in Williamson$'$s synthesis?

What type of alkyl halide are best suited for Williamson ether synthesis?

When a $1^o$ alkyl halide reacts with an alkoxide,what is the product obtained?

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.

The reaction used for the synthesis of ether is

Which of the following on heating gives an ether as the major product?
$(P)$ $C_6H_5CH_2Br + CH_3ONa$
$(Q)$ $C_6H_5ONa + CH_3Br$
$(R)$ $(CH_3)_3CCl + CH_3ONa$
$(S)$ $C_6H_5CH=CHCl + CH_3ONa$