Buna-$N$ synthetic rubber is a copolymer of ........

• A
  $H_2C = CH - CN$ and $H_2C = CH - CH = CH_2$
• B
  $CH_2 = CH - CH = CH_2$ and $CH_2 = CH - CH = CH_2$
• C
  $CH_2 = CH - CN$
• D
  $H_2C = CH - CH = CH_2$ and $H_5C_6 - CH = CH_2$