Regular child care provided outside home or by someone other than the mother does not in itself undermine healthy emotional connections between mothers and their 15-month-old infants, according to a long-term national study. The finding holds even if care begins during the first 3 months after birth and runs for 30 hours or more per week. Among infants who receive unkind and unresponsive care from their mothers, however, the mother-child relationship may be damaged. "This research helps us put apart complexities regarding child care that have not previously been studied in detail," contends Jay Belsky, a psychologist. The investigation consists of 1,153 children and their families living in or near Boston. The youngsters, no more than 1 month old when they entered the study in 1991, will be tracked until the age of 7. Experimenters administered questionnaires to mothers in their homes and videotaped baby caretakers interacting with the kids at ages 1, 6, and 15 months. Independent observers rated the quality of each child care efforts and noted infant nervousness. Unlike most previous studies, this one allows researchers to observe each caretaker’s personality at child nursing, and kids’ emotional reaction by the equipment. The main difference between the investigation and the previous ones is that ______.
A．the researchers started with only one month old infants B．the observers could rate the quality of child care efforts and analyzed them soon C．the researchers were able to give the questionnaires to mothers in their homes D．video equipment enabled researchers to observe what was happening directly