Jonathan is a technologist who has designed a new app. He is convinced that there is a large potential market for the app. Accordingly, he has just quit his current job to focus on developing and marketing the app. Jonathan purchased the rights to plug-in software six months ago and he will use this plug-in software in developing the new app. The cost of this software is: (a) An opportunity cost (b) A sunk cost (c) A differential cost (d) A period cost