15. An unknown hydrocarbon has the molecular formula C$_4$H$_6$. When the unknown, C$_4$H$_6$, is exposed to hydrogenation conditions over a palladium catalyst, two equivalents of H$_2$ are absorbed. If, however, the unknown, C$_4$H$_6$, is exposed to hydrogenation conditions over the Lindlar catalyst, only one equivalent of H$_2$ is absorbed and a disubstituted alkene product is isolated. Propose a reasonable structure for the unknown hydrocarbon.
(4pts)