How many of the following ethers cannot be prepared by willamson’s synthesis ?

$C_6{H}_{5}O{C}_2{H}_5,C_6{H}_{5}OC{H}_3,C_6{H}_{5}C{H}_{2}O{C}_6{H}_5$ $C_6{H}_{5}O{C}_6{H}_5$, ${\left(C{H}_3\right)}_{3}C-O-C{\left(C{H}_3\right)}_3$ ${\left(C{H}_3\right)}_{3}C-O-C{H}_3$

Williamson's synthesis required that the alkyl halide should be 1^∘ and alkoxide ion may be 1^∘,2^∘ or 3^∘ . Thus, two ethers which cannot be prepared by Williamson's synthesis are :$C_6{H}_{5}O{C}_6{H}_5$ and ${\left(C{H}_3\right)}_{3}C-O-C{\left(C{H}_3\right)}_3$