Explain why diene B does not react in a Diels-Alder reaction with the dienophile maleic anhydride; however, diene A engages in a fast and efficient Diels-Alder reaction.

• A. Diene B does not exist in s-cis conformation.
• B. Diene B does not exist in s-trans conformation.
• C. Diene B is not a stable alkene.
• D. Diene B is not electrophilic.

Answer: (A)