Lactose, a sugar found in milk, aids in the absorption of calcium, which in turn is required for bone repair. In addition to there being shortages of milk in tropical areas, inhabitants of these areas lose the ability to absorb lactose, unlike people from nontropical areas. Yet inhabitants of tropical areas have no more problems with bone repair than do people who inhabit nontropical areas. Which one of the following, if true, most helps to resolve the apparent paradox described above? ['People living in tropical areas periodically take tablets containing lactose when there are shortages of milk.', 'Milk consumption has fallen in both tropical and nontropical areas.', 'The extent to which people living in tropical areas lose the ability to absorb lactose depends on a genetic predisposition.', 'The abundant sunlight in tropical areas causes the human body to produce vitamin D naturally, which aids in the absorption of calcium.']