Which of the following can act as the diene component in a Diels-Alder reaction? Select all that apply.

A Diels-Alder diene must be conjugated and able to adopt an s-cis conformation. Buta-1,3-diene can, and the cyclic 1,3-dienes are held s-cis. Penta-1,4-diene is an isolated diene with an $\mathrm{sp}{}_3$ carbon between the double bonds, so it cannot take part.