How many of the following ethers cannot be prepared by Williamson’s synthesis?
$\begin{array}{l}{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{OC}}_2{\mathrm{H}}_5, {\mathrm{C}}_6{\mathrm{H}}_{5}OC{H}_3\\ {\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{OCH}}_2{\mathrm{C}}_6{\mathrm{H}}_5, {\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{OC}}_6{\mathrm{H}}_5,{\left({\mathrm{CH}}_3\right)}_3\mathrm{COC}{\left({\mathrm{CH}}_3\right)}_3,{\left({\mathrm{CH}}_3\right)}_3{\mathrm{COCH}}_3\\ \end{array}$

Williamson’s synthesis required that the alkyl halide should be 10and alkoxide ion may be $1^0,2^0\text{ or }{3}^0$
${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{OC}}_6{\mathrm{H}}_5,{\left({\mathrm{CH}}_3\right)}_3\mathrm{C}-\mathrm{O}-\mathrm{C}{\left({\mathrm{CH}}_3\right)}_3$
cannot be synthesized by Williamson’s Synthesis