单项选择题

Scientists recently revealed an instinct in women untouched and unaffected by the age of technology. Glancing through glossy art books Lee Salk noticed that four times out of five Mary is depicted holding the infant Jesus against her left breast. The Madonna sparked off (bring into action) a series of experiments and observations to determine on which side women hold their babies and why. First he determined that modern mothers still tend to hold their baby on the left. Of 255 right- handed mothers, 83 % held the baby on the left. And out of 32 left-handed women, 78 % held the baby on the left. As a control, women were watched emerging from supermarkets carrying baby-sized packages; the bundles were held with no side preference. Then, dental patients were given a large rubber ball to hold during treatment. The majority clutched the ball to their left side, even when it interfered with the dentist’’s activities. This suggested that in times of tress objects are held against the left side. At that point an apparently contradictory phenomenon was observed. A large number of mothers who brought their premature babies to a follow-up clinic were seen to hold their babies against their right side. So, 115 mothers who had been separated from their babies for 24 hours after birth were observed for holding response. The experimenters presented the baby directly to the midline of the mother’s body, and noted how she held the baby. 53 % placed the baby on the left and 47 % on the right. And it was also noted that the mothers of the group who had held their baby on the left had already had a baby from which they had not been separated after birth. The author suggests that the time immediately after birth is a critical period when the stimulus of holding the baby releases a certain maternal response. That is to say, she senses the baby is better off on her left. Left-handed holding enables the baby to hear the heart-beat-a sound associated with the security of the womb. in order to discover whether hearing heart has a beneficial effect on the baby, the sound of a human heart-beat was played to 102 babies in a New York nursery for 4 days. A control group of babies was not exposed to heart-beats. The babies in the beat group gained markedly more weight and cried far less than the babies in the control group. Salk’’s experiments proved that _____.