Which one of the following statements is not true?

• A
  $Buna-S$ is a copolymer of butadiene and styrene.
• B
  Natural rubber is a $1,4$-polymer of isoprene.
• C
  In vulcanization,the formation of sulphur bridges between different chains makes rubber harder and stronger.
• D
  Natural rubber has the $trans$-configuration at every double bond.