Buna-N synthetic rubber is a copolymer of:

1. A
   $2$ -chloro-$1,3$ -butadiene and $1,3$ -butadiene
2. B
   $1,3$ -butadiene and styrene
3. C
   acrylonitrile and $1,3$ -butadiene
4. D
   acrylonitrile and $2$ -methyl-$1,3$ -butadiene