Nuclear reactors are sometimes built in "geologically quiet" regions, so called by geologists because such regions are distant from plate boundaries and contain only minor faults. Since no minor fault in a geologically quiet region produces an earthquake more often than once in any given 100, 000-year period, it follows that of all potential nuclear reactor sites in such a region, those that are least likely to be struck by an earthquake are ones located near a fault that has produced an earthquake within living memory. Which one of the following is an assumption on which the argument depends? ['For any potential nuclear reactor site, the likelihood of being struck by an earthquake is the primary determinant of site safety.', 'Earthquake faults in geologically quiet regions produce earthquakes at least once in 100, 000 years.', 'In a geologically quiet region, every potential nuclear reactor site is near at least one minor fault.', 'Geologically quiet regions are the least dangerous regions in which to build nuclear reactors.']