$A$ compound reacts with $Cl_2$ to give product $(B)$,and the reaction of product $(B)$ with $(Zn + \text{dilute } HCl)$ gives back the original substance $(A)$. Give the structures of $(A)$ and $(B)$.

(A) The reaction sequence is as follows:
$CH_3CH_3$ $\xrightarrow{Cl_2, h\nu, -HCl} CH_3CH_2Cl$ $\xrightarrow{Zn, H^+, +H_2, -HCl} CH_3CH_3$
Here,$(A)$ is ethane $(CH_3CH_3)$ and $(B)$ is chloroethane $(CH_3CH_2Cl)$.