Cycloheptatrienone (I) is very stable. Cyclopentadienone (II) by contrast is quite unstable and rapidly undergoes a Diels-Alder

Question:

Cycloheptatrienone (I) is very stable. Cyclopentadienone (II) by contrast is quite unstable and rapidly undergoes a Diels-Alder reaction with itself.
(a) Propose an explanation for the different stabilities of these two compounds.
(b) Write the structure of the Diels-Alder adduct of cyclopentadienone.