单选题 By comparing the vocabulary scores with their observations of each child's home life, they were able to conclude that the size of each child's vocabulary correlated most closely to one simple factor: the number of words the parents spoke to the child.
【正确答案】 D
【答案解析】解析：根据句意，可知D选项符合题意。孩子的词汇量与某个因素有关。A选项的含义为“回答的、反应迅速的”；B选项的含义是“敏感的”；C选项的含义是“基本的、重要的”；D的含义是“与某事相关的”。