【解析】定语从句的用法。句中的主语含有一个定语从句，省略了关系代词that，one of the difficulties是作为定语从句中的宾语。have difficulty ( in ) doing sth.做某事有困难，句意“我们学习英语的一个困难是怎样记住新的单词和表达”，因此选B。
【2011 兰州】24. Our Chinese teacher told us interesting story and story was about Thomas Edison.
A. an; a B.the; the C.a; the
D. an; the
答案：D
【解析】冠词的用法。句意“我们的语文老师给我们讲了一个有趣的故事，这个故事是关于托马斯·爱迪生的”，因此选D。
【2011 兰州】25. — How do you like your English teacher?
— He is great. We friends since three years ago.
A. were B. have made C. have been D. have become
答案：C