21. Ron is a semi-retired architect involved in several passive activities. Which of the following is true as to how the passive activity rules apply to Ron and taxpayers in general? a) They must be applied to income or loss from applicable activities before the at-risk rules. b) They apply to a rental activity even if the taxpayer materially participated in that activity. c) They apply to an activity where the taxpayer performs substantial work as an investor and is involved in day-to-day management. d) They apply to a working interest in an oil or gas well in which the taxpayer holds an interest directly or through an entity that does not limit his liability