原文第一段：Unlike so-called basic emotions such as sadness,fear,and anger,guilt emerges a little later,in conjunction with a child’s growing grasp of social and moral norms.Children aren’t born knowing how to say“I’m sorry”;rather,they learn over time that such statements appease parents and friends—and their own consciences.This is why researchers generally regard so-called moral guilt,in the right amount,to be a good thing.

　　题目：21.Researchers think that guilt can be a good thing because it may help______.

　　A）regulate a child's basic emotions

　　B）improve a child's intellectual ability

　　C）foster a child’s moral development

　　D）intensity a child's positive feelings

　　解析：

　　[C]foster a child’s moral development

　　根据题干Researchers think that guilt can be a good thing because it may help______.内容定位到第一段最后一句This is why researchers generally regard so-called moral guilt,in the right amount,to be a good thing.由最后一句的this指代词，可知原因在第一段的前几句，根据第一句Unlike so-called basic emotions such as sadness,fear,and anger,guilt emerges a little later,in conjunction with a child’s growing grasp of social and moral norms.和第二句Children aren’t born knowing how to say“I’m sorry”;rather,they learn over time that such statements appease parents and friends—and their own consciences.可知内疚不是天生的，而是和一个孩子的道德规范有关，是通过后天学习获得的。因此答案是C选项foster a child’s moral development。