Alkyl halide can be converted into alkene by

KI in acetone, undergoes S_N2 reaction with each of P, Q, R and S. The rates of the reaction vary as

In the following reaction sequence, structures of $\mathrm{A}$ and $\mathrm{B}$ are, respectively

The major products of the following reaction are :

The major product in the following reaction is :

The major product of the following reaction is :

Here, $\mathrm{Z}$ is: