Which one of the following reagents are not suitable for the preparation of benzaldehyde from benzene?

a) $\displaystyle \mathrm{CH}_3 \mathrm{Cl}+$ Anhyd. $\displaystyle \mathrm{AlCl}_3$ b) $\displaystyle \mathrm{CrO}_2 \mathrm{Cl}_2+\mathrm{CS}_2 / \mathrm{H}_3 \mathrm{O}^{+}$

a) $\displaystyle \mathrm{CH}_3 \mathrm{Cl}+$ Anhyd. $\displaystyle \mathrm{AlCl}_3$ b) $\displaystyle \mathrm{Cl}_2 / \mathrm{Fe}$ dark $\displaystyle / \mathrm{H}_2 \mathrm{O}(373 \mathrm{~K})$

a) $\displaystyle \mathrm{CO}, \mathrm{HCl}$, Anhyd. $\displaystyle \mathrm{AlCl}_3$

a) $\displaystyle \mathrm{CH}_3 \mathrm{Cl}+\mathrm{Anhyd} . \mathrm{AlCl}_3$ b) $\displaystyle \mathrm{CrO}_3,\left(\mathrm{CH}_3 \mathrm{CO}\right)_2 \mathrm{O}, \mathrm{H}_3 \mathrm{O}^{+}$(Heat)