Animals generally avoid foods that are harmful to them and favor those that benefit them. Thus it is surprising that gypsy moth caterpillars prefer to eat leaves that contain high concentrations of certain naturally occurring toxins called alkaloids, even though caterpillars grow faster on alkaloid-free diets. Which one of the following, if true, most helps to resolve the apparent discrepancy? ['In the alkaloid-containing plants on which gypsy moth caterpillars feed, the toxins are found only in the leaves.', 'The alkaloid-containing plants on which gypsy moth caterpillars feed increase their production of toxins when their leaves are injured.', 'Only gypsy moth caterpillars, not adult gypsy moths, feed on the leaves of alkaloid-containing plants.', "Alkaloids that have been eaten by caterpillars are poisonous to parasitic wasp larvae that feed on caterpillars' internal tissues."]