Adult female rats who have never before encountered rat pups will start to show maternal behaviors after being confined with a pup for about seven days. This period can be considerably shortened by disabling the female' s sense of smell or by removing the scent-producing glands of the pup. Which of the following hypotheses best explains the contrast described above? ['Female rats that have given birth are more affected by olfactory cues than are female rats that have never given birth.', 'A female rat that has given birth shows maternal behavior toward rat pups that she did not bear more quickly than does a female rat that has never given birth.', "The development of a female rat's maternal interest in a rat pup that she did not bear is inhibited by the odor of the pup.", 'The amount of scent produced by rat pups increases when they are in the presence of a female rat that did not bear them.']