Toluene can be easily converted into benzaldehyde by which of the following reagents?

• A

  ${\mathrm{CrO}}_3$ /acetic acid, ${\mathrm{H}}_3{\mathrm{O}}^{+}$
• Correct answer
  B

  ${\mathrm{CrO}}_3$ /acetic anhydride, ${\mathrm{H}}_3{\mathrm{O}}^{+}$
• C

  ${\mathrm{KMnO}}_4/\mathrm{HCl},{\mathrm{H}}_3{\mathrm{O}}^{+}$
• D

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

Hence, the correct answer is option: 'B'.