给定一个包含多个句子的段落，汇总其中出现频率最高的单词。
”Natural language processing (NLP) has seen (and contributed to) a lot of progress in the past few years. One of the most interesting NLP problems is the problem of sentiment analysis, which involves identifying the sentiment (positive, negative or neutral) of a piece of text.”
出现频率最高的单词是 "NLP"。