单选题 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

【正确答案】 D

【答案解析】[注释] 第三段，尤其最后一句提供了本题答案。文中提到与过去不同的是，这次调研使用了录像设备，使研究者可以直接观察照看者与幼儿的情况。