答案：B

题目分析：首先have difficult /trouble doing sth是固定词组。The lecture mainly deals with the trouble young children have distinguishing right from wrong在这个句子中，deal with的宾语是trouble，所以后面的句子是修饰trouble这个词的定语从句。

点评：对于固定用法，平时一定要多归纳、总结和记忆。