There have been no new cases of naturally occurring polio in North America in recent years. Yet there are approximately 12 new cases of polio each year in North America, all caused by the commonly administered live oral polio vaccine (OPV). Substituting inactivated polio vaccine (IPV) for most childhood polio immunizations would cut the number of cases of vaccination-caused polio about in half. Clearly it is time to switch from OPV to IPV as the most commonly used polio vaccine for North American children. Which one of the following, if true, most weakens the argument? ['Although IPV is preferred in some European nations, most countries with comprehensive child immunization programs use OPV.', 'The vast majority of cases of polio caused by OPV have occurred in children with preexisting but unsuspected immunodeficiency disorders.', 'If IPV replaces OPV as the most commonly used polio vaccine, at least a few new cases of naturally occurring polio in North America will result each year.', "A child's risk of contracting polio from OPV has been estimated at 1 in 8. 7 million, which is significantly less than the risk of being struck by lightning."]