Darlene and Ellie are good friends. Ellie has four children, while Darlene does not have any children. Ellie is thinking about giving some property to Darlene. In which of the following parcels would Darlene have an executory interest?

a. A Texas hill country ranch that Ellie conveyed "to Darlene until the first marriage of any of Ellie's children."
b. A downtown building that Ellie conveyed "to the Houston Bar Association for the rest of my life, then to Darlene."
c. A parcel of woodland with hiking trails and picnic areas conveyed "to the City of Houston so long as the land is open to the public for the next 15 years, then to Darlene."
d. A parcel currently used as a parking lot that Ellie's will gives "to the Public Library provided that it is used for library purposes for the next 15 years, then to Darlene."