Reaction of $t-$butyl bromide with sodium methoxide produces

When benzyl chloride is treated with ethanolic $KCN$,the major product formed is

Some alkyl halides undergo substitution whereas some undergo elimination reaction on treatment with bases. Discuss the structural features of alkyl halides with the help of examples which are responsible for this difference.

Compound $(A), \, C_8H_9Br,$ gives a white precipitate when warmed with alcoholic $AgNO_3.$ Oxidation of $(A)$ gives an acid $(B), \, C_8H_6O_4.$ $(B)$ easily forms anhydride on heating. Identify the compound $(A).$

The transition state of the given $S_N2$ reaction is:

What are the products obtained from the following reaction?