Agatha is sentenced to lie imprisonment by state court for felony murder. A few years later, a new law is passed in the state which increases the punishment for felony murder to a death sentence, but no changes are made to Agatha's sentence. Which of the following legal concepts explains why Agatha does not face the death penalty?

a. Bills of attainder
b. Double jeopardy
c. Ex post facto
d. Void-for-vagueness