$($a$)$ "Pyridine is more reactive than benzene towards nucleophilic substitution" Discuss this statement. Ensure that you provide suitable examples and that you show the mechanism involved.

$[12$ marks$]$

$($b$)($i$)$ Identify whether the electrophilic aromatic substitution reaction below shows a primary or secondary kinetic isotope effect and briefly explain how you arrived at your answer.

$($ii$)$ Draw out the mechanism for electrophilic aromatic substitution of benzene and explain why the observed kinetic isotope effect would be the expected result based on this mechanism.

$k\frac{H_H}{k}{D}_D=1\mathrm{.}0$