In the study of chlorination of propane , four products A , B , C and D of the formula $$ C_3H_6Cl_2 $$ were isolated . Each of them was further chlorinated to provide trichloro products , $$ C_3H_5Cl_3 $$ . It was found that A provided one trichloro product , B gave two and C and D each gave three . What are the structural formulae of A , B , C and D ?