Which of the following caspases is an initiator caspase?

The caspase family, now including more than 10 members, can be divided functionally into two groups--initiator and executioner--depending on the order in which they are activated during apoptosis. Initiator caspases include caspase-8 and caspase-9. Several other caspases, including caspase-3 and caspase-6, serve as executioners. Ref: Robbins, 8th edition, Chapter 1.