Scientists have found that female gorilla cubs that are "bred up" by mothers can be replaced by machines that can assume part of the mother's role. When they have offspring, it is difficult to assume the role of mother. Humans and gorillas have great similarities. It shouldn't be managed by a babysitter or daycare center, but should only be raised by their biological mother. Which of the following statements is true to best strengthen the author's conclusion? ['Studies show that individual differences in gorillas are not significant.', 'When female gorillas "bred up" by mothers have offspring, they can be taught some parenting skills.', 'Gorilla cubs reared for a short period of time by their biological mothers can assume the role of mother when they grow up.', 'Gorilla cubs reared by female gorillas other than their biological mothers cannot assume the role of mother when they have offspring.']